V4L/DVB (9903): uvcvideo: V4L2 zoom controls support

Add support for absolute and continuous zoom controls (mapped to absolute
and relative UVC zoom controls).

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