intrng: implement use of intr_isrc_deregister()

[ehem_freebsd_m5p.com]

Summary

With the removal of intrcnt, intr_isrc_deregister() is no longer a
wrapper around panic(). As such in error and release cases,
intr_isrc_deregister() should be called as appropriate.

[ehem_freebsd_m5p.com]

Marking this as paused since I hope to do something about how INTRNG handles interrupt numbers. That could have a larger impact and makes this difficult. Since most interrupt controllers rarely disappear, this isn't too urgent anyway.