drm/panel: rpi-touchscreen: propagate errors in rpi_touchscreen_i2c_read()

There is one caller which checks whether rpi_touchscreen_i2c_read()
returns negative error codes.  Currently it can't because negative
error codes are truncated to u8, but that's easy to fix if we change the
type to int.

Fixes: 2f733d6194bd ("drm/panel: Add support for the Raspberry Pi 7" Touchscreen.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20171020002845.kar2wg7gqxg7tzqi@mwanda
Reviewed-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/panel/panel-raspberrypi-touchscreen.c

@@ -221,7 +221,7 @@ static struct rpi_touchscreen *panel_to_ts(struct drm_panel *panel)
 	return container_of(panel, struct rpi_touchscreen, base);
 }
 
-static u8 rpi_touchscreen_i2c_read(struct rpi_touchscreen *ts, u8 reg)
+static int rpi_touchscreen_i2c_read(struct rpi_touchscreen *ts, u8 reg)
 {
 	return i2c_smbus_read_byte_data(ts->i2c, reg);
 }