[MAC80211]: kill key_mgmt, use key_management_enabled

The key_mgmt variable for STA interfaces doesn't seem well-defined
nor do we actually use the values other than "NONE", so change it to
be named better.

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