Fix GIC interrupt decoding in INTRNG code

Bug was already fixed in not-INTRNG code, it needs to be corrected
here as well.  Source: https://reviews.freebsd.org/rS294422

Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: cognet, wma
Approved by: cognet (mentor)
Differential Revision: https://reviews.freebsd.org/D5029