media: i2c: imx290: Split control initialization to separate function

The imx290_probe() function is too large. Split control initialzation to
a dedicated function to increase code readability.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>