iio: buffer: fix the function signature to match implementation

linux/iio/buffer-dma.h was not updated to when length was changed to
unsigned int.

Fixes: c043ec1ca5ba ("iio:buffer: make length types match kfifo types")
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/linux/iio/buffer-dma.h

@@ -141,7 +141,7 @@ int iio_dma_buffer_read(struct iio_buffer *buffer, size_t n,
 	char __user *user_buffer);
 size_t iio_dma_buffer_data_available(struct iio_buffer *buffer);
 int iio_dma_buffer_set_bytes_per_datum(struct iio_buffer *buffer, size_t bpd);
-int iio_dma_buffer_set_length(struct iio_buffer *buffer, int length);
+int iio_dma_buffer_set_length(struct iio_buffer *buffer, unsigned int length);
 int iio_dma_buffer_request_update(struct iio_buffer *buffer);
 
 int iio_dma_buffer_init(struct iio_dma_buffer_queue *queue,