[media] smiapp: Fix resource management in registration failure

If the registered() callback failed, resources were left unaccounted for.
Fix this, as well as add unregistering the sub-devices in driver
unregistered() callback.

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>