HID: multitouch: Add suffix for HID_DG_TOUCHPAD

Instead of printing "UNKNOWN" as device suffix for HID_DG_TOUCHPAD call
the device as "Touchpad".

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Daniel Martin <consume.noise@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>