Give the SPI module a chance of working...

[pjsg]

Fixes #3494.

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

There were a number of things broken about the spi support.

• The parameter to transfer for the address was actually addr rather than address
• If specified, the cs line has to be configured as a gpio and set to the inactive value
• The txdata argument to transfer didn't work.

This PR fixes all of the above and it appears to work (at least it drives the one device that I have tried it on!)

[marcelstoer]

requested a review from marcelstoer

Sorry Philip, I wish I did understand that code. I guess @jmattsson should take a look instead.

[jmattsson]

Other than the documentation issue Marcel picked up on, this looks good as far as I can see. Feel free to merge once that command->cmd has been adressed!