Increase I2C clock speed and decrease debounce time

Increase the I2C clock speed to a higher reasonable value of 400kHz.
Also, since the Ergodox takes roughly 3 ms per scan, there is no reason
to have an excessively long debounce time. 2 scans is enough.

Both of these changes have the effect of allowing higher consistent
typing speeds.

It turns out these changes are already in your cub_layout branch, so this pull request is basically just porting them to master. I think it would be a good idea to do so; I based my firmware off master, and had a mediocre typing experience until I made these changes.