Well, to be honest I’m not familiar with the Ahead, but what normally happens when people can’t make special peripherals work, is that the PINMUX registers are set to the wrong function.
If you can drop the SCL line with a gpioset $(gpiofind p9.19)=0
then the function is set to MODE 7, which isn’t what you want.
All this PINMUXing can be a little convoluted, seeing how it keeps coming up,
but a resent thread discusses this point at length and can be found here.
You can also go to the device-trees for the BBB and find inspiration for what you need.