[media] smiapp: Move sub-device initialisation into a separate function

Simplify smiapp_init() by moving the initialisation of individual
sub-devices to a separate function.

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>