The 82c54 model (pit.cc) implements timer modes 0, 2, and 3 in its handler
functions, without caring which timer number is involved. However, the
I/O write code that sets the mode is inconsistent.
Timer 0 can be set to modes 0,2,3 only.
Timer 1 can be set to mode 2 only.
Timer 2 can be set to mode 2,3 only.
From a quick reading of an 8254 datasheet, I can't see any reason to
restrict which timer can be in which mode, so I think it's correct to
allow ALL timers to go into ALL modes that are implemented.
