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

commit af05ef01e9cde84620c6855a8d8ab9c8a1db9009 upstream.

This fixes a regression introduced by commit f7059ea 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>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>