Input: wacom - prepare the driver to include BT devices

Now that wacom is a hid driver, there is no point in having a separate
driver for bluetooth devices. This patch prepares the common paths of
Bluetooth devices in the common wacom driver. It also adds the sysfs file
"speed" used by Bluetooth devices.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Tested-by: Przemo Firszt <przemo@firszt.eu>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>