nl80211: limit to one pairwise cipher for associate()

In this case, only one cipher makes sense, unlike for
connect() where it may be possible to have the card or
driver select.

No changes to mac80211 due to the way the structs are
laid out -- but the loop in net/mac80211/cfg.c will
degrade to just zero or one passes.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>