Input: wacom - fix wacom->shared guards for dual input devices

features->quirks can have multiple bits set. For dual input, we only
need to check WACOM_QUIRK_MULTI_INPUT.

Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>