staging: iio: tsl2x7x_core: Fix standard deviation calculation

Standard deviation is calculated as the square root of the variance
where variance is the mean of sample_sum and length. Correct the
computation of statP->stddev in accordance to the proper calculation.

Fixes: 3c97c08b ("staging: iio: add TAOS tsl2x7x driver")
Reported-by: Abhiram Balasubramanian <abhiram@cs.utah.edu>
Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/light/tsl2x7x_core.c

@@ -854,7 +854,7 @@ void tsl2x7x_prox_calculate(int *data, int length,
 		tmp = data[i] - statP->mean;
 		sample_sum += tmp * tmp;
 	}
-	statP->stddev = int_sqrt((long)sample_sum) / length;
+	statP->stddev = int_sqrt((long)sample_sum / length);
 }
 
 /**