[media] v4l: Unify selection flags

Unify flags on the selection interfaces on V4L2 and V4L2 subdev. Flags are
very similar to targets in this case: there are more similarities than
differences between the two interfaces.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>