[media] usbtv: fix dependency

This fixes a dependency problem as found by Randy Dunlap:
https://lkml.org/lkml/2013/6/27/501

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>