[media] cx25821-alsa: get rid of a __must_check warning

[media] cx25821-alsa: get rid of a __must_check warning

The hole reason for __must_check is to not ignore an error.

However, a "ret" value is used at cx25821 just to avoid the
Kernel compilation to compain about it.

That, however, produces another warning (with W=1):

drivers/media/pci/cx25821/cx25821-alsa.c: In function 'cx25821_audio_fini':
drivers/media/pci/cx25821/cx25821-alsa.c:727:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]

With the current implementation of driver_for_each_device() and
cx25821_alsa_exit_callback(), there's actually just one
very unlikely condition where it will currently produce
an error: if driver_find() returns NULL.

Ok, there's not much that can be done, as it is on a driver's
function that returns void, but it can at least print some message
if the error happens.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>