iio: buffer: dmaengine: unwrap the use of iio_buffer_set_attrs()

The iio_buffer_set_attrs() helper will be removed in this series. So, just
assign the attributes of the DMAEngine buffer logic directly.

This is IIO buffer core context, so there is direct access to the
buffer->attrs object.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200929125949.69934-2-alexandru.ardelean@analog.com


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