ath9k: Fix a NULL pointer dereference in ath_rate_get

It looks like mac80211 may try to send unicast frames to a STA that
does not have a STA entry. We need to make sure that that is caught in
the rate control code before dereferencing STA data.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>