[media] usbtv: split core and video implementation

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>