V4L/DVB (8194): gspca: Fix the format of the low resolution mode of spca561.

The low (half) res modes of the spca561 are not spca561 compressed, but are
raw bayer, this patches fixes this and adds a PIX_FMT define for the GBRG
bayer format used by the spca561 in low res mode.

Signed-off-by: Hans de Goede <j.w.r.degoede@hhs.nl>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>