Add configuration function to enable or disable automatic retransmit

[samcrow]

In some cases, we don't want the CAN peripheral to repeatedly try to transmit again each frame that cannot be transmitted. With this pull request, I am proposing a small change that adds a function to CanConfig to enable or disable automatic retransmission using the NART bit.

[jonas-schievink]

Thanks, this looks good! Can you extend the docs to mention that automatic retransmit is enabled by default?

[samcrow]

I added a note to the set_automatic_retransmit documentation. is that good?

[jonas-schievink]

Yup, thanks!

bors r+

[bors[bot]]

Build succeeded:

• ci