[net80211] Migrate short slot time configuration into per-vap and deferred taskqueue updates.

The 11b/11g ERP and slot time update handling are two things which weren't
migrated into the per-VAP state when Sam did the initial VAP work.
That makes sense for a lot of setups where net80211 is driving radio state
and the radio only cares about the shared state.

However, as noted by a now deleted comment, the ERP and slot time updates
aren't EXACTLY correct/accurate - they only take into account the most
RECENTLY created VAP, and the state updates when one creates/destroys
VAPs isn't exactly great.

So:

• track the short slot logic per VAP;
• whenever the slot time configuration changes, just push it into a deferred task queue update so drivers don't have to serialise it themselves;
• if a driver registers a per-VAP slot time handler then it'll just get the per VAP one;
• .. if a driver registers a global one then the legacy behaviour is maintained - a single slot time is calculated and pushed out.

Note that the calculated slot time is better than the existing logic - if ANY
of the VAPs require long slot then it's disabled for all VAPs rather than
whatever the last configured VAP did.

Now, this isn't entirely complete - the rest of ERP tracking around short/long
slot capable station tracking needs to be converted into per-VAP, as well
as the preamble/barker flags. Luckily those also can be done in a similar
fashion - keep per-VAP counters/flags and unify them before doing the driver
update. I'll defer that work until later.

All the existing drivers can keep doing what they're doing with the global
slot time flags as that is maintained. One driver (iwi) used the per-VAP
flags instead of the ic flags, so now that driver will work properly.

This unblocks some ath10k porting work as the firmware takes the slot time
configuration per-VAP rather than globally, and some firmware handles
STA+AP and STA+STA (on same/different channels) configurations where
the firmware will switch slot time as appropriate.

Tested:

• AR9380, STA/AP mode
• AR9880 (ath10k), STA mode