mac80211: clarify interface iteration and make it configurable

During hardware restart, all interfaces are iterated even
though they haven't been re-added to the driver, document
this behaviour. The same also happens during resume, which
is even more confusing since all of the interfaces were
previously removed from the driver. Make this optional so
drivers relying on the current behaviour can still use it,
but to let drivers that don't want this behaviour disable
it.

Also convert all API users, keeping the old semantics
except in hwsim, where the new normal ones are desired.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>