media: ov5675: use group write to update digital gain

MWB gain register are used to set gain for each mwb channel mannually.
However, it will involve some artifacts at low light environment as gain
cannot be applied to each channel synchronously. Update the driver to use
group write for digital gain to make the sure RGB digital gain be 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@kernel.org>