stm32: BusOFF, BusPassive, BusWarning removed from curr_error in FDCAN and BXCAN

[NeoHylde]

Changes:

• curr_error doesn't read PSR.BO/EW/EP.

Problem:
When awaiting can::read() / try_read(), if the CAN controller enters ErrorWarning or ErrorPassive, the future resolves immediately if the RxFifo's are empty and the CAN controller is in ErrorPassive or ErrorWarning.

For async functions this can result in tight polling loops that:

• hog cpu
• starve other async tasks

Root cause is treating a bus error state (BusPassive, BusWarning, BusOff) as an error. Error states reflect the TEC or REC registers exceeding certain numbers, while the LEC is the error last detected on the bus. Additionally, LEC may be "reset" on read, EW, EP, and BO do not.

Fix:
Read no longer reads BO, EW, EP registers in PSR registers.

Possible Future Changes
I would recommend seperating the error checks from the can read function, and creating a dedicated error checking function that is exposed through properties.

Meaning you can detect errors outside of reads, giving a bit more versatility.