HID: wacom: Don't register pad_input for touch switch

Touch switch state is received through WACOM_PAD_FIELD. However, it
is reported by touch_input. Don't register pad_input if no other pad
events require the interface.

Cc: stable@vger.kernel.org
Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>