HID: wacom: generic: Reset events back to zero when pen leaves

As a pen leaves, we need to be sure to reset all events back to zero
so that userspace is able to get the complete pen state when it enters
proximity again.

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