rndis_wlan: do not set default_key if not WEP key

rndis_set_default_key did call add_wep_key to set default key on device, even
if key is WPA. This caused rndis_wlan not work with wpa_supplicant in nl80211
mode (causing disconnect from AP).

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>