ad4630 fails dma_rx test

[Trecia Agoylo]

When running test_ad4630.py, dma_rx test failed with the following:

    def rx(self):
>       if not self._rx__rxbuf:
E       AttributeError: 'ad4630' object has no attribute '_rx__rxbuf'

I attached full test result.
ad4630-test-result.txt