[media] v4l2 core: improve debug flag handling

The old debug field is renamed to dev_debug to ensure that existing drivers
(including out-of-tree drivers) that try to use the old name will no longer
compile. A comment has also been added that makes it explicit that drivers
shouldn't use this field.

Additional bits have been added to the debug flag to be more fine-grained
when debugging, especially when dealing with streaming ioctls and read,
write and poll. You want to enable those explicitly to prevent flooding
the log when streaming unless you actually want to do that.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>