Revert "intrng: change multi-interrupt root support type to enum"

This reverts commit 536c8d948e8563141356fd41fb8bfe65be289385. The
change seemed fine on the surface, but converting to an enum has raised
some concerns due to the asm <-> C interface. Back it out and let
someone else deal with it later if they'd like to.

Further context about the concerns can be found in D47279.