Turn off Wire I2C mode on keepon poweroff

[qdot]

If the keepon is powered off, we need to kick back into bootup mode so we can pull the bus pins low. Unfortunately, there's no Wire.end() call that will call twi_close() for us, meaning that twi still has its interrupts set on next keepon powerup and we can't pull the lines to stop the bus from communicating. We need to be able to stop the interrupts and go back to "normal".