wifi: mac80211: debugfs: lock wiphy instead of RTNL

Since we no longer really use the RTNL, there's no point
in locking it here. Most drivers don't really need to
have any locks here anyway, and the rest are probably
completely broken, but it's a debugfs-only callback so
it really doesn't matter much.

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