[PATCH] input: alps: fix compatibility with -funsigned-char

[msizanoen]

The AlpsPS/2 code previously relied on the assumption that `char` is a
signed type, which was true on x86 platforms (the only place where this
driver is used) before kernel 6.2. However, on 6.2 and later, this
assumption is broken due to the introduction of -funsigned-char as a new
global compiler flag.

Fix this by explicitly specifying the signedness of `char` when sign
extending the values received from the device.

Fixes: f3f33c677699 ("Input: alps - Rushmore and v7 resolution support")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: msizanoen <msizanoen@xxxxxxxxxxx>
---
 drivers/input/mouse/alps.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/input/mouse/alps.c b/drivers/input/mouse/alps.c
index 989228b5a0a4..1c570d373b30 100644
--- a/drivers/input/mouse/alps.c
+++ b/drivers/input/mouse/alps.c
@@ -2294,20 +2294,20 @@ static int alps_get_v3_v7_resolution(struct psmouse *psmouse, int reg_pitch)
 	if (reg < 0)
 		return reg;
 
-	x_pitch = (char)(reg << 4) >> 4; /* sign extend lower 4 bits */
+	x_pitch = (signed char)(reg << 4) >> 4; /* sign extend lower 4 bits */
 	x_pitch = 50 + 2 * x_pitch; /* In 0.1 mm units */
 
-	y_pitch = (char)reg >> 4; /* sign extend upper 4 bits */
+	y_pitch = (signed char)reg >> 4; /* sign extend upper 4 bits */
 	y_pitch = 36 + 2 * y_pitch; /* In 0.1 mm units */
 
 	reg = alps_command_mode_read_reg(psmouse, reg_pitch + 1);
 	if (reg < 0)
 		return reg;
 
-	x_electrode = (char)(reg << 4) >> 4; /* sign extend lower 4 bits */
+	x_electrode = (signed char)(reg << 4) >> 4; /* sign extend lower 4 bits */
 	x_electrode = 17 + x_electrode;
 
-	y_electrode = (char)reg >> 4; /* sign extend upper 4 bits */
+	y_electrode = (signed char)reg >> 4; /* sign extend upper 4 bits */
 	y_electrode = 13 + y_electrode;
 
 	x_phys = x_pitch * (x_electrode - 1); /* In 0.1 mm units */
-- 
2.39.2