ath9k: use the passed ieee80211_hw on ath_rx_prepare()

this now uses the proper hw which should mean finding the
right sta when using ath9k virtual wiphy stuff. Only
advantage I see here is getting the rssi properly updated
so the 'fix' itself isn't that great, but at least this
is correct.

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>