wifi: mac80211: ethtool: always hold wiphy mutex

Drivers should really be able to rely on the wiphy mutex
being held all the time, unless otherwise documented. For
ethtool, that wasn't quite right. Fix and clarify this in
both code and documentation.

Reported-by:  <syzbot+c12a771b218dcbba32e1@syzkaller.appspotmail.com>
Fixes: 0e8185ce ("wifi: mac80211: check wiphy mutex in ops")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>