Fix MPIC mask/unmask

Before the fix for single interrupt, both percpu and non-percpu routes
were enabled/disable at the same time.

Submitted by: Marcin Wojtas <mw@semihalf.com
Obtained from: Semihalf
Sponsored by: Stormshield, Netgate
Reviewed by: loos
Differential revision: https://reviews.freebsd.org/D10716