[media] smiapp: Split off sub-device registration into two

Remove the loop in sub-device registration and create each sub-device
explicitly instead.

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>