Input: wacom - split out the pad device for Bamboos

We rely on the return code of wacom_bpt*() to do the input_sync().
wacom_wac_irq() then properly sync the input devices.

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