v4 is what is left from the broadcast-IRQ series, plus yet another two new
changes. Logically the changes are largely independent, and can likely go in
in about any order.

1: avoid indirect call to event handler
2: make another channel flags update atomic
3: move legacy tick IRQ count adjustment
4: reduce hpet_next_event() call sites
5: drop "long timeout" handling from reprogram_hpet_evt_channel()
6: simplify "expire" check a little in reprogram_hpet_evt_channel()
7: drop .set_affinity hook
8: don't arbitrarily cap delta in reprogram_hpet_evt_channel()

Jan