The "protection needed" flag is currently parsed out of the ERP IE in
beacons. This patch allows the ERP IE to be available at assocation time
and causes the appropriate actions to be performed earlier.

It is slightly complicated by the fact that most APs don't include the
ERP IE in association responses. To work around this, we store ERP
values in the ieee80211_sta_bss structure.

Also added some WLAN_ERP defines for use by upcoming patches.

Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>