wifi: mac80211: reduce iflist_mtx

We now hold the wiphy mutex everywhere that we use or
needed the iflist_mtx, so we don't need this mutex any
more in mac80211. However, drivers may also iterate,
and in some cases (e.g. mt76) do so from high-priority
contexts. Thus, keep the mutex around but remove its
usage in mac80211 apart from those driver-visible parts
that are still needed.

Most of this change was done automatically with spatch,
with the parts that are still needed as described above
reverted manually.

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