mac80211: correct ieee80211_iterate_active_interfaces_mtx() locking comments

mac80211: correct ieee80211_iterate_active_interfaces_mtx() locking comments

Commit a05829a7222e ("cfg80211: avoid holding the RTNL when calling the
driver") dropped usage of RTNL here and replaced it with
hw->wiphy->mutex. But we didn't update the comments.

Fixes: a05829a7222e ("cfg80211: avoid holding the RTNL when calling the driver")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Link: https://lore.kernel.org/r/20210505202829.1039400-1-briannorris@chromium.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>