[media] v4l2-ioctl: fill in the description for VIDIOC_ENUM_FMT

The descriptions used in drivers for the formats returned with ENUM_FMT
are all over the place.

So instead allow the core to fill them in if the driver didn't. This
allows drivers to drop the description and flags.

Based on an earlier patch from Philipp Zabel:
http://comments.gmane.org/gmane.linux.drivers.video-input-infrastructure/81411Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>