media: i2c: max9286: Break-out reverse channel setup

Break out the reverse channel setup configuration procedure to its own
function.

This change prepares for configuring the reverse channel conditionally
to the remote side high threshold configuration.

No functional changes intended.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesasa@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>