Revert "intrng: switch from MAXCPU to mp_ncpus"

This reverts commit b4d11915c73f199501672b278be86e1f63790036.
This is a direct commit to stable/14. The change breaks booting
on older Raspberry Pi 4's, although that works on main. The cause
is unknown. The original commit should be redone on stable/14
if/when it catches up with main.

(cherry picked from commit 3e627553bbd791a4f73eaeea2c2d795fd4e0ee70)

Approved-by: re (cperciva)