hdac_attach() registers an interrupt handler before allocating various
driver resources which are accessed by the interrupt handler. On some
platforms we observe what appear to be spurious interrupts upon a cold
boot, resulting in panics.

Partially work around the problem by deferring irq allocation until
after other resources are allocated. I think this is not a complete
solution, but is correct and sufficient to work around the problems
reported in the PR.

PR: 268393