V4L/DVB (13026): s2250-board: Implement brightness and contrast controls

The brightness and contrast controls were added to the Sensoray 2250 device.
A read register function was added to set the correct bit fields.

Signed-off-by: Pete Eberlein <pete@sensoray.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>