ath10k: fix null deref on wmi-tlv when trying spectral scan

ath10k: fix null deref on wmi-tlv when trying spectral scan

WMI ops wrappers did not properly check for null
function pointers for spectral scan. This caused
null dereference crash with WMI-TLV based firmware
which doesn't implement spectral scan.

The crash could be triggered with:

  ip link set dev wlan0 up
  echo background > /sys/kernel/debug/ieee80211/phy0/ath10k/spectral_scan_ctl

The crash looked like this:

  [  168.031989] BUG: unable to handle kernel NULL pointer dereference at           (null)
  [  168.037406] IP: [<          (null)>]           (null)
  [  168.040395] PGD cdd4067 PUD fa0f067 PMD 0
  [  168.043303] Oops: 0010 [#1] SMP
  [  168.045377] Modules linked in: ath10k_pci(O) ath10k_core(O) ath mac80211 cfg80211 [last unloaded: cfg80211]
  [  168.051560] CPU: 1 PID: 1380 Comm: bash Tainted: G        W  O    4.8.0 #78
  [  168.054336] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
  [  168.059183] task: ffff88000c460c00 task.stack: ffff88000d4bc000
  [  168.061736] RIP: 0010:[<0000000000000000>]  [<          (null)>]           (null)
  ...
  [  168.100620] Call Trace:
  [  168.101910]  [<ffffffffa03b9566>] ? ath10k_spectral_scan_config+0x96/0x200 [ath10k_core]
  [  168.104871]  [<ffffffff811386e2>] ? filemap_fault+0xb2/0x4a0
  [  168.106696]  [<ffffffffa03b97e6>] write_file_spec_scan_ctl+0x116/0x280 [ath10k_core]
  [  168.109618]  [<ffffffff812da3a1>] full_proxy_write+0x51/0x80
  [  168.111443]  [<ffffffff811957b8>] __vfs_write+0x28/0x120
  [  168.113090]  [<ffffffff812f1a2d>] ? security_file_permission+0x3d/0xc0
  [  168.114932]  [<ffffffff8109b912>] ? percpu_down_read+0x12/0x60
  [  168.116680]  [<ffffffff811965f8>] vfs_write+0xb8/0x1a0
  [  168.118293]  [<ffffffff81197966>] SyS_write+0x46/0xa0
  [  168.119912]  [<ffffffff818f2972>] entry_SYSCALL_64_fastpath+0x1a/0xa4
  [  168.121737] Code:  Bad RIP value.
  [  168.123318] RIP  [<          (null)>]           (null)

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>