Input: wacom - fix mousewheel handling for old wacom tablets

This fixes a regression introduced in
3b57ca0f80c5c8994b5b1e3d3f904cfe727951f2.

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>

drivers/input/tablet/wacom_wac.c

@@ -243,10 +243,10 @@ static int wacom_graphire_irq(struct wacom_wac *wacom)
 			if (features->type == WACOM_G4 ||
 					features->type == WACOM_MO) {
 				input_report_abs(input, ABS_DISTANCE, data[6] & 0x3f);
-				rw = (signed)(data[7] & 0x04) - (data[7] & 0x03);
+				rw = (data[7] & 0x04) - (data[7] & 0x03);
 			} else {
 				input_report_abs(input, ABS_DISTANCE, data[7] & 0x3f);
-				rw = -(signed)data[6];
+				rw = -(signed char)data[6];
 			}
 			input_report_rel(input, REL_WHEEL, rw);
 		}