Immediately after clearing a pending callout that didn't make it due

Immediately after clearing a pending callout that didn't make it due
to the lock we hold, disable interrupts, and announce to the firmware
that we are shutting down. Especially do this before disabling blocks.

This makes some types of machines with asf enabled no longer hang upon
boot, when we start configuring the interface.

PR:			i386/96382, kern/100410, kern/122252, kern/116328
Reported by:		erwin
Hardware provided by:	TDC A/S
Reviewed by:		stas
Tested by:		stas