[media] smiapp: Drop BUG_ON() in suspend path

Checking that the mutex is not acquired is unnecessary for user processes
are stopped by this point. Drop the check.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>