V4L/DVB: gspca - sn9c20x: Bad transfer size of Bayer images

As the end of frame marker does not contain any pixel, it must not be
transferred.

Reported-by: Ivo Jager <ivo.jager@siliconfields.net>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>