media: i2c: imx219: Simplify format assignment

The set_fmt and init_cfg functions both fills a v4l2_mbus_framefmt
instance, passing in the mode and the media bus code. While set_fmt
uses function helpers, init_cfg open-codes the assignments.

Simplify the format initialization by moving it to a common helper.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>