HID: wacom: Avoid sending empty sync events

Empty sync events clutter up logs and are a waste of CPU cycles. We can
avoid sending mt_sync events if touch is disabled or a specific slot is
unused. We can avoid sending full sync events if no events were generated.

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