media/i2c: ov7251: Switch from V4L2_CID_GAIN to V4L2_CID_ANALOGUE_GAIN

The mainline driver has implemented analogue gain using the control
V4L2_CID_GAIN instead of V4L2_CID_ANALOGUE_GAIN.

libcamera requires V4L2_CID_ANALOGUE_GAIN, and therefore fails.

Update the driver to use V4L2_CID_ANALOGUE_GAIN.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>