[media] gspca_main: Update buffer flags even when user_copy fails

Before this patch dqbuf errors out on a failing user_copy (with user pointers)
before updating the buffer flags, causing a successsfully dequeued buffer
to still have the DONE flag, which means that it could no longer be
re-queueud (assuming the app somehow survives the segfault).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>