Input: wacom - split out the pad device for the wireless receiver

The Wireless Receiver should also behave in the same way than regular
USB devices.

To simplify the unregistering of the different devices,
wacom_unregister_inputs() is introduced.
For consistency, the function wacom_register_input() is renamed into
wacom_register_inputs().

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>