[media] v4l2-ctrls: drivers should be able to ignore the READ_ONLY flag

When applications try to set READ_ONLY controls an error should
be returned. However, when drivers do that it should be accepted.

Those controls could reflect some driver status which the application
can't change but the driver obviously has to be able to change it.

This is needed among others for future HDMI status controls.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>