[WIRELESS] cfg80211: Clarify locking comment.

This patch clarifies the comment about locking in wiphy_unregister.

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>