ath9k: fix crash with WEP in ad-hoc mode

Commit eed8e22f added support for using
multicast key lookup to support per-vif/sta keys for AP and ad-hoc.
Unfortunately, it also introduced a crash in ad-hoc mode when the sta
pointer is NULL, which happens when setting up an interface with WEP
keys. This patch fixes it by falling back to the assigned key index.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Reported-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>