Does not work with more than one card

Thanks for your work.
I wanted to use more than one card, so I took your code and improved on it.
See https://pypi.org/project/waveshare-d2a-a2d-pigpio/
and https://github.com/the-moog/waveshare_a2d_d2a_python

Happy for you to pull my changes or leave the fork as-is.