Hooked up a bit differently to the other tools. Those tools can be static for
all and be re-used. The wacom driver initializes the pad with the correct
number of buttons though and we can't do this until we have the pad done event.

If the tablet is removed and we plug a different one in, we should initialize
that correctly, so unlike the other tools the pad is properly removed and
re-initialized on plug.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Ping Cheng <ping.cheng@wacom.com>