iio: buffer: dmaengine: obtain buffer object from attribute

The reference to the IIO buffer object is stored on the attribute object.
So we need to unwind it to obtain it.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-16-alexandru.ardelean@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>