Input: wacom - fix pressure in Cintiq 21UX2

Currently the pressure range in Cintiq 21UX2 is limited to half of the
supported. This patch fixes the problem.

Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
CC: stable@kernel.org
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>