I2c mode doesn't appear to work

[3rdIteration]

I'm looking at adapting this for a raspberry pi hat and got a few of the square boards fabricated as a test.

The UART functionality works, but not i2c, and when looking at the documentation and some other designs, I think the communication mode pins are backwards.

I'm happy to make a tweak, have it fabbed, test and bring a PR if that helps.

[revk]

I have only ever tested UART as it seems fast enough (most time is not in the comms).

But if just wrong way around it just needs labels changed I assume.