Input: appletouch - fix integer overflow issue

When reading data from Geyser 2 touchpads used on post Oct 2005 Apple
PowerBooks the driver was casting X and Y coordinates values to
'signed char'. Testing on one of such PowerBooks I have noticed that
touchpad always generates positive values, but some of them are greater
that 127, and thus, when cast to 'signed char' being interpreted as
a negative.

Such bigger values have been observed infrequently, closer to the
edges of a touchpad, so the problem was not very visible.
Nevertheless, the patch would potentially improve touchpad
driver accuracy.

Signed-off-by: Vadim Zaliva <lord@crocodile.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/mouse/appletouch.c

@@ -205,8 +205,8 @@ struct atp {
 	bool			overflow_warned;
 	int			x_old;		/* last reported x/y, */
 	int			y_old;		/* used for smoothing */
-	signed char		xy_cur[ATP_XSENSORS + ATP_YSENSORS];
-	signed char		xy_old[ATP_XSENSORS + ATP_YSENSORS];
+	u8			xy_cur[ATP_XSENSORS + ATP_YSENSORS];
+	u8			xy_old[ATP_XSENSORS + ATP_YSENSORS];
 	int			xy_acc[ATP_XSENSORS + ATP_YSENSORS];
 	int			idlecount;	/* number of empty packets */
 	struct work_struct	work;
@@ -531,7 +531,7 @@ static void atp_complete_geyser_1_2(struct urb *urb)
 
 	for (i = 0; i < ATP_XSENSORS + ATP_YSENSORS; i++) {
 		/* accumulate the change */
-		signed char change = dev->xy_old[i] - dev->xy_cur[i];
+		int change = dev->xy_old[i] - dev->xy_cur[i];
 		dev->xy_acc[i] -= change;
 
 		/* prevent down drifting */