V4L/DVB: uvcvideo: Make button controls work properly

According to the v4l2 spec, writing any value to a button control should
result in the action belonging to the button control being triggered.
UVC cams however want to see a 1 written, this patch fixes this by
overriding whatever value user space passed in with -1 (0xffffffff) when
the control is a button control.

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