mac80211: use IW_AUTH_PRIVACY_INVOKED rather than IW_AUTH_KEY_MGMT

In the long bug-hunt for why dynamic WEP networks didn't work it
turned out that mac80211 incorrectly uses IW_AUTH_KEY_MGMT while
it should use IW_AUTH_PRIVACY_INVOKED to determine whether to
associate to protected networks or not.

This patch changes the behaviour to be that way and clarifies the
existing code.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>