V4L/DVB (8372): gspca: Small ov6650 fixes.

1) Don't change  the red and blue pre-gain's from their defaults
2) Actually make the powerline freq ctrl available to userspace

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>