platform/x86: dell-ddv: Fix temperature scaling

[ Upstream commit 0331b1b0ba ]

After using the built-in UEFI hardware diagnostics to compare
the measured battery temperature, i noticed that the temperature
is actually expressed in tenth degree kelvin, similar to the
SBS-Data standard. For example, a value of 2992 is displayed as
26 degrees celsius.
Fix the scaling so that the correct values are being displayed.

Tested on a Dell Inspiron 3505.

Fixes: a77272c160 ("platform/x86: dell: Add new dell-wmi-ddv driver")
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20230218115318.20662-2-W_Armin@gmx.de
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/platform/x86/dell/dell-wmi-ddv.c

@@ -14,7 +14,6 @@
 #include <linux/errno.h>
 #include <linux/kernel.h>
 #include <linux/kstrtox.h>
-#include <linux/math.h>
 #include <linux/module.h>
 #include <linux/limits.h>
 #include <linux/power_supply.h>
@@ -192,7 +191,8 @@ static ssize_t temp_show(struct device *dev, struct device_attribute *attr, char
 	if (ret < 0)
 		return ret;
 
-	return sysfs_emit(buf, "%d\n", DIV_ROUND_CLOSEST(value, 10));
+	/* Use 2731 instead of 2731.5 to avoid unnecessary rounding */
+	return sysfs_emit(buf, "%d\n", value - 2731);
 }
 
 static ssize_t eppid_show(struct device *dev, struct device_attribute *attr, char *buf)