V4L/DVB: gspca - sq930x: Change the gain value for Micron sensors

The bit 7 of the sensor gain register multiplies the 7bits value by two.
It is now always set with half the user gain value.

Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>