mac80211: don't read ERP information from (re)association response

According to the standard, the field cannot be present, so don't
try to interpret it either.

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