wifi: mac80211: clean up band switch in duration

Most devices now do duration calculations, so we don't hit
this code at all any more. Clearly the approach of warning
at compile time here when new bands are added didn't work,
the new bands were just added with "TODO". Clean it up, it
won't matter for new bands since they'll just not have any
need to calculate durations in software.

While at it, also clean up and unify the code a bit.

Link: https://msgid.link/20240129194108.70a97bd69265.Icdd8b0ac60a382244466510090eb0f5868151f39@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>