Convert the i2c register accesses to utilize the CCI helper library
rather than relying on driver-specific functions. Also, set analogue
gain in a single 16-bit write instead of two 8-bit writes.

[Sakari Ailus: Initialise sensor_gain as 0.]
Signed-off-by: Mikhail Rudenko <mike.rudenko@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>