drivers: thermal: tsens: fix potential integer overflow on multiply

Currently a multiply operation is being performed on two int values
and the result is being assigned to a u64, presumably because the
end result is expected to be probably larger than an int. However,
because the multiply is an int multiply one can get overflow. Avoid
the overflow by casting degc to a u64 to force a u64 multiply.

Also use div_u64 for the divide as suggested by Daniel Lezcano.

Addresses-Coverity: ("Unintentional integer overflow")
Fixes: fbfe1a042cfd ("drivers: thermal: tsens: Add interrupt support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Link: https://lore.kernel.org/r/20191101100035.25502-1-colin.king@canonical.com

drivers/thermal/qcom/tsens-common.c

@@ -92,7 +92,7 @@ void compute_intercept_slope(struct tsens_priv *priv, u32 *p1,
 
 static inline u32 degc_to_code(int degc, const struct tsens_sensor *s)
 {
-	u64 code = (degc * s->slope + s->offset) / SLOPE_FACTOR;
+	u64 code = div_u64(((u64)degc * s->slope + s->offset), SLOPE_FACTOR);
 
 	pr_debug("%s: raw_code: 0x%llx, degc:%d\n", __func__, code, degc);
 	return clamp_val(code, THRESHOLD_MIN_ADC_CODE, THRESHOLD_MAX_ADC_CODE);