Ensure all filters are deactivated before changing any.

This doesn't fix the problems I'm having but it does make it really
clear that STM32F4 Reference Manual section 32.9.1 "Register access
protection" is accounted for.
Reset any pending status interrupts at init.

Not really necessary, but avoids confusion and questions if you look at
the CAN peripheral status register later.
Bugfix: put CAN_FiRx16 fields in the right order.
bsp-can-test: turn LED on initially.

This way it's easy to tell if you've built it for the right platform.
Bugfix: extended IDs discard low 18 bits for `stid` field.