HID: wacom: Report ABS_TILT_{X,Y} as signed values

Centers the ABS_TILT_{X,Y} axes so that a value of zero is reported when
the pen is vertical. Combined with resolution information in the next
patch, this makes it possible for userspace to calculate the pen angle
without needing hardware-specific knowledge. The xf86-input-wacom driver
was updated to support signed tilt values in late-2012 (2f2acec).

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>