IIO: bmp280-core.c: fix error in humidity calculation

While calculating the compensation of the humidity there are negative values
interpreted as unsigned because of unsigned variables used.  These values as
well as the constants need to be casted to signed as indicated by the
documentation of the sensor.
Signed-off-by: NAndreas Klinger <ak@it-klinger.de>
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Reviewed-by: NMatt Ranostay <matt.ranostay@konsulko.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/pressure/bmp280-core.c

@@ -175,11 +175,12 @@ static u32 bmp280_compensate_humidity(struct bmp280_data *data,
 	}
 	H6 = sign_extend32(tmp, 7);
 
-	var = ((s32)data->t_fine) - 76800;
-	var = ((((adc_humidity << 14) - (H4 << 20) - (H5 * var)) + 16384) >> 15)
-		* (((((((var * H6) >> 10) * (((var * H3) >> 11) + 32768)) >> 10)
-		+ 2097152) * H2 + 8192) >> 14);
-	var -= ((((var >> 15) * (var >> 15)) >> 7) * H1) >> 4;
+	var = ((s32)data->t_fine) - (s32)76800;
+	var = ((((adc_humidity << 14) - (H4 << 20) - (H5 * var))
+		+ (s32)16384) >> 15) * (((((((var * H6) >> 10)
+		* (((var * (s32)H3) >> 11) + (s32)32768)) >> 10)
+		+ (s32)2097152) * H2 + 8192) >> 14);
+	var -= ((((var >> 15) * (var >> 15)) >> 7) * (s32)H1) >> 4;
 
 	return var >> 12;
 };