Fix 8259 IRQ priority resolver.
Initialize the 8259 such that IRQ7 is the lowest priority.

Reviewed by:		tychon
Differential Revision:	https://reviews.freebsd.org/D1322
MFC after:		1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275850 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f