SERCOM changes break I2C

> This hunk (BUSERR-> ERROR) breaks I2C communication. Wire::endTransmission() returns "received NACK on transmit of address" 

 _Originally posted by @kaysievers in [ccfc7db](https://github.com/adafruit/ArduinoCore-samd/commit/ccfc7db988fe2f61dd3f8734794fa9f69730f58d#r166294191)_

Thanks!