media: i2c: Switch control to V4L2_CID_ANALOGUE_GAIN

The V4L2_CID_GAIN control for this driver configures registers that
the datasheet specifies as analogue gain. Switch the control's ID
to V4L2_CID_ANALOGUE_GAIN.

Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>