cfg80211: Clear encryption privacy when key off is done.

When the current_bss is not set, 'iwconfig <iface> key off' does not
clear the private flag. Hence after we connect with WEP to an AP and
then try to connect with another non-WEP AP, it does not work.
This issue will not be seen if supplicant is used.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>