Return the spurious interrupt vector (IRQ7 or IRQ15) if the atpic cannot
find any unmasked pin with an interrupt asserted.

Reviewed by:	tychon
CR:		https://reviews.freebsd.org/D669
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270434 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f