iio: buffer-dmaengine: add dev-managed calls for buffer alloc

Currently, when using a 'iio_dmaengine_buffer_alloc()', an matching call to
'iio_dmaengine_buffer_free()' must be made.

With this change, this can be avoided by using
'devm_iio_dmaengine_buffer_alloc()'. The buffer will get free'd via the
device's devres handling.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>