Don't call enable_all_rings if the adapter has been freed.

This is a subtle use-after-free race that results in some very undesirable
hang behaviour.

Reviewed by: pkelsey
Obtained from: Kip Macy, NextBSD (https://github.com/NextBSD/NextBSD/commit/91a9bd1dbb33dafb41684d054e59d73976de9654)