pwc: Add support for control events

Since the pwc driver already uses v4l2-device, v4l2-fh, and the control
framework, all that is needed is hooking up event subscription.

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