ath9k: simpify RX by calling ath_get_virt_hw() once

ath_get_virt_hw() is required on RX to determine for which virtual
wiphy an skb came in for. Instead of searching for the hw twice do
it only once.

Cc: Jouni.Malinen <Jouni.Malinen@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>