[media] pwc: Make auto white balance speed and delay available as v4l2 controls

Currently auto white balance speed and delay are only available through custom
ioctls, which are deprecated and will be going away in 3.3 .

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>