thermal/drivers/rcar_gen3: Fix undefined temperature if negative

As description for DIV_ROUND_CLOSEST in file include/linux/kernel.h.
  "Result is undefined for negative divisors if the dividend variable
   type is unsigned and for negative dividends if the divisor variable
   type is unsigned."

In current code, the FIXPT_DIV uses DIV_ROUND_CLOSEST but has not
checked sign of divisor before using. It makes undefined temperature
value in case the value is negative.

This patch fixes to satisfy DIV_ROUND_CLOSEST description
and fix bug too. Note that the variable name "reg" is not good
because it should be the same type as rcar_gen3_thermal_read().
However, it's better to rename the "reg" in a further patch as
cleanup.

Signed-off-by: Van Do <van.do.xw@renesas.com>
Signed-off-by: Dien Pham <dien.pham.ry@renesas.com>
[shimoda: minor fixes, add Fixes tag]
Fixes: 564e73d283 ("thermal: rcar_gen3_thermal: Add R-Car Gen3 thermal driver")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Niklas Soderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Niklas Soderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1593085099-2057-1-git-send-email-yoshihiro.shimoda.uh@renesas.com

drivers/thermal/rcar_gen3_thermal.c

@@ -167,7 +167,7 @@ static int rcar_gen3_thermal_get_temp(void *devdata, int *temp)
 {
 	struct rcar_gen3_thermal_tsc *tsc = devdata;
 	int mcelsius, val;
-	u32 reg;
+	int reg;
 
 	/* Read register and convert to mili Celsius */
 	reg = rcar_gen3_thermal_read(tsc, REG_GEN3_TEMP) & CTEMP_MASK;