media: i2c: imx290: Limit analogue gain according to module

The imx327 only supports up to 29.4dB of analogue gain, vs
the imx290 going up to 30dB. Both are in 0.3dB steps.

As we now have model specific config, fix this mismatch,
and delete the comment referencing it.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>