CAN2 (PB5/PB6) doesn't work on STM32F429

[terraphantm]

Ran into this while messing with running my own transceiver on an octopus pro using alternative pins. I get no communication on PB5/PB6 with CanBoot, but klipper works fine. I suspect it is similar to the issue that resulted in this klipper commit: https://github.com/Klipper3d/klipper/pull/5988/commits

Edit: I never realized the can code is more or less the same as in klipper. I copied the revised can.c from that klipper commit to my local CanBoot install and recompiled. Now it works as expected.