[media] gspca - zc3xx: Discard the partial frames

In some cases, some frames may not end with the JPEG end of frame.
Being not complete, they are now discarded.

Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>