staging:iio:ad2s90: Make read_raw return spi_read's error code

Previously, when spi_read returned an error code inside ad2s90_read_raw,
the code was ignored and IIO_VAL_INT was returned. This patch makes the
function return the error code returned by spi_read when it fails.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s90.c

@@ -36,11 +36,12 @@ static int ad2s90_read_raw(struct iio_dev *indio_dev,
 
 	mutex_lock(&st->lock);
 	ret = spi_read(st->sdev, st->rx, 2);
-	if (ret)
-		goto error_ret;
+	if (ret < 0) {
+		mutex_unlock(&st->lock);
+		return ret;
+	}
 	*val = (((u16)(st->rx[0])) << 4) | ((st->rx[1] & 0xF0) >> 4);
 
-error_ret:
 	mutex_unlock(&st->lock);
 
 	return IIO_VAL_INT;