HID: multitouch: Fix system-control buttons not working

Some laptops have system-control buttons (e.g. KEY_SLEEP) on the same
interface as a hid-multitouch touch-pad.

This commit fixes these buttons not working.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>