HID: wacom: generic: support touch on/off softkey

Wacom Cintiq Pro has a softkey to turn touch on/off. Since it is
a softkey, hardware/firmware still reports touch events no matter
what state the softkey is. We need to ignore touch events when
the key is in off mode.

Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>