Fixes for the LIN master task behaviour

Aggressively reinit the LIN block before sending a new frame header.
Rename methods to clarify master/slave task use.
Avoid passing the current FID around as an argument, since it can be
fetched from a register when needed.
Adjust the soft serial timing value to get serial into spec on the master board

Not clear whether this is due to compiler changes, or just a bad board
oscillator…