iio: chemical: bme680: Fix overflows in compensate() functions

There are cases in the compensate functions of the driver that
there could be overflows of variables due to bit shifting ops.
These implications were initially discussed here [1] and they
were mentioned in log message of Commit 1b3bd8592780 ("iio:
chemical: Add support for Bosch BME680 sensor").

[1]: https://lore.kernel.org/linux-iio/20180728114028.3c1bbe81@archlinux/

Fixes: 1b3bd8592780 ("iio: chemical: Add support for Bosch BME680 sensor")
Signed-off-by: Vasileios Amoiridis <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/chemical/bme680_core.c

···
 	if (!calib->par_t2)
 		bme680_read_calib(data, calib);
 
-	var1 = (adc_temp >> 3) - (calib->par_t1 << 1);
+	var1 = (adc_temp >> 3) - ((s32)calib->par_t1 << 1);
 	var2 = (var1 * calib->par_t2) >> 11;
 	var3 = ((var1 >> 1) * (var1 >> 1)) >> 12;
-	var3 = (var3 * (calib->par_t3 << 4)) >> 14;
+	var3 = (var3 * ((s32)calib->par_t3 << 4)) >> 14;
 	data->t_fine = var2 + var3;
 	calc_temp = (data->t_fine * 5 + 128) >> 8;
 
···
 	var1 = (data->t_fine >> 1) - 64000;
 	var2 = ((((var1 >> 2) * (var1 >> 2)) >> 11) * calib->par_p6) >> 2;
 	var2 = var2 + (var1 * calib->par_p5 << 1);
-	var2 = (var2 >> 2) + (calib->par_p4 << 16);
+	var2 = (var2 >> 2) + ((s32)calib->par_p4 << 16);
 	var1 = (((((var1 >> 2) * (var1 >> 2)) >> 13) *
-			(calib->par_p3 << 5)) >> 3) +
+			((s32)calib->par_p3 << 5)) >> 3) +
 			((calib->par_p2 * var1) >> 1);
 	var1 = var1 >> 18;
 	var1 = ((32768 + var1) * calib->par_p1) >> 15;
···
 	var3 = ((press_comp >> 8) * (press_comp >> 8) *
 			(press_comp >> 8) * calib->par_p10) >> 17;
 
-	press_comp += (var1 + var2 + var3 + (calib->par_p7 << 7)) >> 4;
+	press_comp += (var1 + var2 + var3 + ((s32)calib->par_p7 << 7)) >> 4;
 
 	return press_comp;
 }
···
 		 (((temp_scaled * ((temp_scaled * calib->par_h5) / 100))
 		   >> 6) / 100) + (1 << 14))) >> 10;
 	var3 = var1 * var2;
-	var4 = calib->par_h6 << 7;
+	var4 = (s32)calib->par_h6 << 7;
 	var4 = (var4 + ((temp_scaled * calib->par_h7) / 100)) >> 4;
 	var5 = ((var3 >> 14) * (var3 >> 14)) >> 10;
 	var6 = (var4 * var5) >> 1;