[media] smiapp: Unify setting up sub-devices

The initialisation of the source sub-device is somewhat different as it's
not created by the smiapp driver itself. Remove redundancy in initialising
the two kind of sub-devices.

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>