[WIRELESS] cfg80211: Update comment for locking.

This patch adds a comment that was part of my rtnl locking patch for
cfg80211 but which I forgot for the merge.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>