V4L/DVB (7905): check_v4l2 should return -EINVAL on error

check_v4l2 always returns 0, so this change is an noop for now,
but a comment says it will return something else in the future

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>