airo: fix IW_AUTH_ALG_OPEN_SYSTEM

IW_AUTH_ALG_OPEN_SYSTEM is ambiguous in set_auth for WEP as
wpa_supplicant uses it for both no encryption and WEP open system.
Cache the last mode set (only of these two) and use it here.

This allows wpa_supplicant to work with unencrypted APs.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>