Input: wacom - use new input-mt routines

This patch brings wacom driver in-sync with input-mt changes
made in release 3.7.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>