[media] V4L: mx2_camera: .try_fmt shouldn't fail

If the user is requesting too large a frame, instead of failing
select an acceptable geometry, preserving the requested aspect ratio.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>