media: bcm2835-unicam: Disable event-related ioctls on metadata node

The unicam driver supports both the SOURCE_CHANGE and CTRL events. Both
events are only generated on the image video node, so the event-related
ioctls are useless on the medatada node. Disable them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>