staging:iio:ad5933: Fix NULL pointer deref when enabling buffer

staging:iio:ad5933: Fix NULL pointer deref when enabling buffer

In older versions of the IIO framework it was possible to pass a
completely different set of channels to iio_buffer_register() as the one
that is assigned to the IIO device. Commit 959d2952d124 ("staging:iio: make
iio_sw_buffer_preenable much more general.") introduced a restriction that
requires that the set of channels that is passed to iio_buffer_register() is
a subset of the channels assigned to the IIO device as the IIO core will use
the list of channels that is assigned to the device to lookup a channel by
scan index in iio_compute_scan_bytes(). If it can not find the channel the
function will crash. This patch fixes the issue by making sure that the same
set of channels is assigned to the IIO device and passed to
iio_buffer_register().

Fixes the follow NULL pointer derefernce kernel crash:
	Unable to handle kernel NULL pointer dereference at virtual address 00000016
	pgd = d53d0000
	[00000016] *pgd=1534e831, *pte=00000000, *ppte=00000000
	Internal error: Oops: 17 [#1] PREEMPT SMP ARM
	Modules linked in:
	CPU: 1 PID: 1626 Comm: bash Not tainted 3.15.0-19969-g2a180eb-dirty #9545
	task: d6c124c0 ti: d539a000 task.ti: d539a000
	PC is at iio_compute_scan_bytes+0x34/0xa8
	LR is at iio_compute_scan_bytes+0x34/0xa8
	pc : [<c03052e4>]    lr : [<c03052e4>]    psr: 60070013
	sp : d539beb8  ip : 00000001  fp : 00000000
	r10: 00000002  r9 : 00000000  r8 : 00000001
	r7 : 00000000  r6 : d6dc8800  r5 : d7571000  r4 : 00000002
	r3 : d7571000  r2 : 00000044  r1 : 00000001  r0 : 00000000
	Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
	Control: 18c5387d  Table: 153d004a  DAC: 00000015
	Process bash (pid: 1626, stack limit = 0xd539a240)
	Stack: (0xd539beb8 to 0xd539c000)
	bea0:                                                       c02fc0e4 d7571000
	bec0: d76c1640 d6dc8800 d757117c 00000000 d757112c c0305b04 d76c1690 d76c1640
	bee0: d7571188 00000002 00000000 d7571000 d539a000 00000000 000dd1c8 c0305d54
	bf00: d7571010 0160b868 00000002 c69d3900 d7573278 d7573308 c69d3900 c01ece90
	bf20: 00000002 c0103fac c0103f6c d539bf88 00000002 c69d3b00 c69d3b0c c0103468
	bf40: 00000000 00000000 d7694a00 00000002 000af408 d539bf88 c000dd84 c00b2f94
	bf60: d7694a00 000af408 00000002 d7694a00 d7694a00 00000002 000af408 c000dd84
	bf80: 00000000 c00b32d0 00000000 00000000 00000002 b6f1aa78 00000002 000af408
	bfa0: 00000004 c000dc00 b6f1aa78 00000002 00000001 000af408 00000002 00000000
	bfc0: b6f1aa78 00000002 000af408 00000004 be806a4c 000a6094 00000000 000dd1c8
	bfe0: 00000000 be8069cc b6e8ab77 b6ec125c 40070010 00000001 22940489 154a5007
	[<c03052e4>] (iio_compute_scan_bytes) from [<c0305b04>] (__iio_update_buffers+0x248/0x438)
	[<c0305b04>] (__iio_update_buffers) from [<c0305d54>] (iio_buffer_store_enable+0x60/0x7c)
	[<c0305d54>] (iio_buffer_store_enable) from [<c01ece90>] (dev_attr_store+0x18/0x24)
	[<c01ece90>] (dev_attr_store) from [<c0103fac>] (sysfs_kf_write+0x40/0x4c)
	[<c0103fac>] (sysfs_kf_write) from [<c0103468>] (kernfs_fop_write+0x110/0x154)
	[<c0103468>] (kernfs_fop_write) from [<c00b2f94>] (vfs_write+0xd0/0x160)
	[<c00b2f94>] (vfs_write) from [<c00b32d0>] (SyS_write+0x40/0x78)
	[<c00b32d0>] (SyS_write) from [<c000dc00>] (ret_fast_syscall+0x0/0x30)
	Code: ea00000e e1a01008 e1a00005 ebfff6fc (e5d0a016)

Fixes: 959d2952d124 ("staging:iio: make iio_sw_buffer_preenable much more general.")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>