media: ov7670: Add the get_fmt callback

Add the get_fmt callback, also enable V4L2_SUBDEV_FL_HAS_DEVNODE flag
to make this subdev has device node.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>