ath9k: fix using sta->drv_priv before initializing it

A station pointer can be passed to the driver on tx, before it has been
marked as associated. Since ath9k_sta_state was initializing the entry
too late, it resulted in some spurious crashes.

Fixes: df3c6eb3 ("ath9k: Use sta_state() callback")
Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>