[media] gspca - sn9c20x: Fix loss of frame start

The frame header was checked on packets of size 64 bytes only, while the webcams
may put a frame header at the beginning of bigger packets.

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