cfg80211: fix disabling WPA via wext (SIOCSIWAUTH)

cfg80211_set_wpa_version completely missed the use case when disabling
WPA, considering IW_AUTH_WPA_VERSION_DISABLED an invalid argument. This
caused weird error messages in wpa_supplicant.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>