V4L/DVB (12704): gspca - sn9c20x: Fix exposure on SOI968 sensors

Fixes broken exposure on SOI968 webcams that was causing
the camera to display a black screen

Signed-off-by: Brian Johnson <brijohn@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>