net80211: simplify an #ifdef INET/INET6 block

[bz]

Summary

I got lost in the #ifdef #endif here so I checked 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
MFC after: 5 days

Test Plan

I would highly appreciate if people would carefully
double-check that I manually change-ifdefed it correctly.

[zlei]

sys/net80211/ieee80211_output.c

1289–1291

Missing break here.

1307

ditto

[bz]

Add missing breaks. Thank you for care fully checking! :)

[zlei]

Looks good to me.