HID: wacom: generic: Don't return a value for wacom_wac_event

It is unnecessary to return a value since nothing is expecting a
value from it.

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>