media: ov13858: Increase digital gain granularity, range

Previously, possible digital gains were just 1x, 2x and 4x. These
coarse gains were not sufficient in fine-tuning the image capture.

Now, digital gain range is [0, 16x] with each step 1/1024, default 1x.
This is achieved through OV13858 MWB R/G/B gain controls.

Signed-off-by: Chiranjeevi Rapolu <chiranjeevi.rapolu@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>