HID: wacom: generic: Send data only when the interface is defined

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>