[media] gspca-core: Fix buffers staying in queued state after a stream_off

This fixes a regression introduced by commit f7059eaa and should be
backported to all supported stable kernels which have this commit.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Antonio Ospite <ospite@studenti.unina.it>
CC: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>