Input: wacom - fix mousewheel handling for old wacom tablets

This fixes a regression introduced in
3b57ca0f.

The data[6] byte contains either 1 or -1 depending on the whether the
mouse wheel on older wacom tablets is moved down (1) or up (-1).  The
patch introduced in the above commit changed the cast from (signed char)
to (signed).  When cast as a signed integer and negated, the value of -1
(stored in the byte as 0xff) became -255 rather than 1.  This patch
reverts the cast to a (signed char) and also removes an unnecessary
(signed) cast, as all the values operated on are bitmasked.

Signed-off-by: Mike Auty <ikelos@gentoo.org>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Cc; stable@kernel.org
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>