Input: wacom - make one-bit signed bitfields unsigned

Otherwise it can only take the values 0/-1 which doesn't seem to
have been intended.

drivers/input/tablet/wacom.h:108:12: error: dubious one-bit signed bitfield

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>