This isnt perfect, there is a race left somewhere still but its closer.

Make drive->id not NULL an invariant
Clean up a few things from that
hwif specific queue length
initialisation/IRQ cleanups

Note: this changes the default blocks limit per I/O to 256. I've still seen
no credible evidence that its a problem and "other OS's" do it.

This is the first of a set of changes to make DRIVER(drive)!=NULL an
invariant

Yes NEC use *different* function numbers!!

[There are a ton of updates to pull from 2.4, but not yet merged]