media: ov9734: use group write for digital gain

As the RGB digital gains of ov9734 were not applied as group, some
artifacts were observed in low light environment, use group write for
digital gain can make the RGB digital can be guaranteed to applied
together at frame boundary.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>