staging:iio:spear_adc: Remove unused variable

Remove the scale_mv variable from the read_raw() callback. Fixes the following
warning:
	drivers/staging/iio/adc/spear_adc.c: In function 'spear_read_raw':
	drivers/staging/iio/adc/spear_adc.c:149:6: warning: unused variable 'scale_mv'
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/spear_adc.c

@@ -146,7 +146,6 @@ static int spear_read_raw(struct iio_dev *indio_dev,
 			  long mask)
 {
 	struct spear_adc_info *info = iio_priv(indio_dev);
-	u32 scale_mv;
 	u32 status;
 
 	switch (mask) {