Videopix Frame Grabber: Fix unreleased lock in vfc_debug()

Videopix Frame Grabber: vfc_debug() doesn't release the device lock
when copy_from_user() fails

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Acked-by: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>