iio: dma-buffer: Cleanup buffer.h/buffer_impl.h includes

The IIO DMA buffer is a DMA buffer implementation. As such it should
include buffer_impl.h rather than buffer.h.

The include to buffer.h in buffer-dma.h should be buffer_impl.h so it has
access to the struct iio_buffer definition. The code currently only works
because all places that use buffer-dma.h include buffer_impl.h before it.

The include to buffer.h in industrialio-buffer-dma.c  can be removed since
those file does not reference any of buffer consumer functions.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>