V4L/DVB (13506): uvcvideo: Factorize common field in uvc_entity structure

The bNrInPins and baSourceID fields are common among all entities (some
of use bSourceID but this is conceptually the same). Move those two
fields out of entity type-specific unions into the uvc_entity structure
top level.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>