i2c: use open drain for output rather than flipping between input and output.

[ourairquality]

Using gpio_enable and flipping the configuration appears to be a major cause of poor performance in the i2c driver, so this changes to using the outputs as open-drain only.

[Zaltora]

Great fix, the performance gain is significant. I did a test with DS3231 example with a delay of 1 us from sdk :
old i2c lib = > 90kHz ; 1787 us process time
fix i2c lib => 300 kHz ; 940 us process time (current fix #214)
fast i2c lib => 370 kHz ; 570 us process time ( the alternative lib #202 )

[projectgus]

Thanks @ourairquality, this looks good to me! 👍

[sheinz]

Thanks @ourairquality.

Has anyone tried GPIO 16 with i2c already?

[ourairquality]

@sheinz It's not expected for work on GPIO 16 now as GPIO 16 has no open drain support (or at least it's not in the code yet).

[Zaltora]

One other inconvenient of GPIO 16, this pin is very slow to switch between state.