[MAC80211]: ignore key index on pairwise key (WEP only)

Work-around for broken APs that use a non-zero key index for WEP
pairwise keys. With this patch, WEP encryption only is exempt from
providing a zero key index.

Signed-off-by: Volker Braun <volker.braun@physik.hu-berlin.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>