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: c043ec1c ("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>