media: rcar-csi2: set default format if a unsupported one is requested

Instead of failing the set_fmt() if a unsupported format is requested
set a default one and return the changed format to the user.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reported-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>