[media] V4L: soc_camera: allow reading from video device if supported

Try reading on video device. If the camera bus driver supports reading
we can try it and return the result. Also add a debug line.

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