nl80211: add missing parameter clearing

Jouni and Maxim reported an oops when using wpa_supplicant -Dnl80211,
which seems to be due to random data being contained in the crypto
settings for the assoc() command. This seems to be due to the missing
memset here, so add it -- it's certainly missing but I'm not 100%
certain that it will fix the problem.

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