media: i2c: max9286: Configure reverse channel amplitude

Adjust the initial reverse channel amplitude parsing from
firmware interface the 'maxim,reverse-channel-microvolt'
property.

This change is required for both rdacm20 and rdacm21 camera
modules to be correctly probed when used in combination with
the max9286 deserializer.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>