[media] smiapp: Split sub-device initialisation off from the registered callback

The registered callback is called by the V4L2 async framework after the
bound callback. This allows separating the functionality in the registered
callback so that on DT based systems only sub-device registration is done
there.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>