iio: buffer: fix the function signature to match implementation

[ Upstream commit 92397a6c38d139d50fabbe9e2dc09b61d53b2377 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>