staging: bcm2835-audio: Disconnect and free vchi_instance on module_exit()

staging: bcm2835-audio: Disconnect and free vchi_instance on module_exit()

In the current implementation, vchi_instance is inited during the first
call of bcm2835_audio_open_connection(), and is never freed. It causes a
memory leak when the module `snd_bcm2835` is removed.

Here is how this commit fixes it:

* the VCHI context (including vchi_instance) is created once in the
  platform's devres
* the VCHI context is allocated and connected once during module_init()
* all created bcm2835_chips have a pointer to this VCHI context
* bcm2835_audio_open_connection() can access the VCHI context through the
  associated bcm2835_chip
* the VCHI context is disconnected and freed once during module_exit()

After this commit is applied, I don't see other issues with the module's
init/exit, so I also remove the associated TODO task.

Steps to reproduce the memory leak before this commit:

~~~~
root@raspberrypi:/home/pi# aplay test0.wav
Playing WAVE 'test0.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Ster
^CAborted by signal Interrupt...
root@raspberrypi:/home/pi# rmmod snd_bcm2835
root@raspberrypi:/home/pi# modprobe snd_bcm2835
root@raspberrypi:/home/pi# aplay test0.wav
Playing WAVE 'test0.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Ster
^CAborted by signal Interrupt...
root@raspberrypi:/home/pi# echo scan > /sys/kernel/debug/kmemleak
root@raspberrypi:/home/pi# cat /sys/kernel/debug/kmemleak
unreferenced object 0xb6794c00 (size 128):
  comm "aplay", pid 406, jiffies 36870 (age 116.650s)
  hex dump (first 32 bytes):
    08 a5 82 81 01 00 00 00 08 4c 79 b6 08 4c 79 b6  .........Ly..Ly.
    00 00 00 00 00 00 00 00 ad 4e ad de ff ff ff ff  .........N......
  backtrace:
    [<802af5e0>] kmem_cache_alloc_trace+0x294/0x3d0
    [<806ce620>] vchiq_initialise+0x98/0x1b0
    [<806d0b34>] vchi_initialise+0x24/0x34
    [<7f1311ec>] 0x7f1311ec
    [<7f1303bc>] 0x7f1303bc
    [<7f130590>] 0x7f130590
    [<7f111fd8>] snd_pcm_open_substream+0x68/0xc4 [snd_pcm]
    [<7f112108>] snd_pcm_open+0xd4/0x248 [snd_pcm]
    [<7f112334>] snd_pcm_playback_open+0x4c/0x6c [snd_pcm]
    [<7f0e250c>] snd_open+0xa8/0x14c [snd]
    [<802ce590>] chrdev_open+0xac/0x188
    [<802c57b4>] do_dentry_open+0x10c/0x314
    [<802c6ba8>] vfs_open+0x5c/0x88
    [<802d9a68>] path_openat+0x368/0x944
    [<802dacd4>] do_filp_open+0x70/0xc4
    [<802c6f70>] do_sys_open+0x110/0x1d4
~~~~

Signed-off-by: Kirill Marinushkin <k.marinushkin@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-rpi-kernel@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: devel@driverdev.osuosl.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>