V4L/DVB (13385): gspca - pac7302: Add red and blue balance control.

The valid values for these controls are 0..3 (lower two bits) which was
identified by trial and error on Labtec Webcam 2200 (USB ID 093a:2626).
The upper 6 bits are ignored on page 0, registers 0xc5 and 0xc7 by the camera.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>