Optimize CAN RX FIFO usage in CAN filters

[heliochronix]

The current implementation of the CAN filters uses only FIFO0 to receive messages. We should optimize usage of the FIFOs to ensure a single FIFO doesn't get overwhelmed and drop received frames.

One FIFO should probably be used for high priority critical messages, while another can be used for lower priority PDO and SDO frames and such.