staging:iio:ad7793: Add missing break in switch statement

Without the break statement we fall right through to the default case and return
an error value.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/ad7793.c

@@ -676,7 +676,7 @@ static int ad7793_write_raw(struct iio_dev *indio_dev,
 				}
 				ret = 0;
 			}
-
+		break;
 	default:
 		ret = -EINVAL;
 	}