mac80211: handle -EALREADY on cfg80211 op assoc req

When the SME requests to associate to an open AP
ieee80211_sta_set_extra_ie() can be called with zero IE
length. When this happens or when the extra IE has already
been set -EALREADY is passed down and the supplicant will
complain that the operation is already in progress and it will
not let us associate. We correct this by treating -EALREADY
from ieee80211_sta_set_extra_ie() as a success just as we do
for wext.

Cc: Shan.Palanisamy@Atheros.com
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>