HID: wacom: make the WL connection friendly for the desktop

Currently, tablets connected to the WL receiver all share the same
VID/PID. There is no way for the user space to know which one is which
besides parsing the name. We can force the PID to be set to the
actual hardware. This way, the input device will have the correct PID
which can be match in libwacom.

With only this trick, the pad input does not inherit the ID_INPUT_TABLET
udev property from its parent. We can force udev to accept it by declaring
a BTN_STYLUS which is never used.

This way, tablets connected through WL can be used from the user point of
view in the same way they are used while connected through wire.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>