iio: inkern: fix a static checker error

Avoid this smatch error:
drivers/iio/inkern.c:751 iio_read_avail_channel_raw() error: double unlock 'mutex:&chan->indio_dev->info_exist_lock'

Fixes: 00c5f80c ("iio: inkern: add helpers to query available values from channels")
Signed-off-by: NPeter Rosin <peda@axentia.se>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/inkern.c

@@ -750,11 +750,9 @@ int iio_read_avail_channel_raw(struct iio_channel *chan,
 err_unlock:
 	mutex_unlock(&chan->indio_dev->info_exist_lock);
 
-	if (ret >= 0 && type != IIO_VAL_INT) {
+	if (ret >= 0 && type != IIO_VAL_INT)
 		/* raw values are assumed to be IIO_VAL_INT */
 		ret = -EINVAL;
-		goto err_unlock;
-	}
 
 	return ret;
 }