Fix problem with WEP unicast key > index 0

The functions ieee80211_wx_{get,set}_encodeext fail if one tries to set
unicast (IW_ENCODE_EXT_GROUP_KEY not set) keys at key indices>0. But at
least some Cisco APs dish out dynamic WEP unicast keys at index !=0.

Signed-off-by: Volker Braun <volker.braun@physik.hu-berlin.de>
Signed-off-by: James Ketrenos <jketreno@linux.intel.com>