HID: wacom: Interpret tilt data from Intuos Pro BT as signed values

The tilt data contained in the Bluetooth packets of an Intuos Pro are
supposed to be interpreted as signed values. Simply casting the values
to type `char` is not guaranteed to work since it is implementation-
defined whether it is signed or unsigned. At least one user has noticed
the data being reported incorrectly on their system. To ensure that the
data is interpreted properly, we specifically cast to `signed char`
instead.

Link: #445
Fixes: 4922cd26f03c ("HID: wacom: Support 2nd-gen Intuos Pro's Bluetooth classic interface")
CC: stable@vger.kernel.org # 4.11+
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>
[jason.gerecke@wacom.com: Imported into input-wacom (49a397ad24ee)]
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>

4.18/wacom_wac.c

@@ -1373,9 +1373,9 @@ static void wacom_intuos_pro2_bt_pen(struct wacom_wac *wacom)
 					rotation -= 1800;
 
 				input_report_abs(pen_input, ABS_TILT_X,
-						 (char)frame[7]);
+						 (signed char)frame[7]);
 				input_report_abs(pen_input, ABS_TILT_Y,
-						 (char)frame[8]);
+						 (signed char)frame[8]);
 				input_report_abs(pen_input, ABS_Z, rotation);
 				input_report_abs(pen_input, ABS_WHEEL,
 						 get_unaligned_le16(&frame[11]));