Fix i2c write

[9names]

I2C write was unreliable for a few reasons.

1. timeouts were very short
2. we were only waiting for i2c_bus_busy to be cleared, which sometimes happens before the fifo is empty.

By switching timeouts to use elapsed microseconds (estimated via McycleDelay) and waiting for an empty fifo, I can reliably use a SSD1306 screen now. This was impossible before.

Resolves #48