V4L/DVB: gspca_sonixb: leave bridge gain at 1.0 when we have a sensor gain

Leave bridge gain at 1.0 when we have a sensor gain, while at it also
change the bridge gain setting code to write both registers in one go
saving us a usb transaction.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>