Sometime valid events may not be supported by the driver yet. Make
sure we don't process them when the code is not ready.

This fix prevents a kernel panic due to unsupported HID events.
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>