net80211: simplify an #ifdef INET/INET6 block

I got lost in the #ifdef #endif here so I changed the code to a
switch block with two non-overlapping #ifdef parts and a default
which makes it a lot easier to read.

No functional changes.

Sponsored by: The FreeBSD Foundation
Reviewed by: (zlei.huang gmail.com)
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D36411