Input: wacom - fix serial number handling on Cintiq 21UX2

Cintiq 21UX2 added 8 more bits for the tool serial number and more
buttons for the expresskey. We did not enable them properly in the
last patch.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>