ath9k: use the ieee80211_hw to get to an sband on ath_rx_prepare()

No need to use the private driver structure to get to an sband.
This will make it easier to share this code with ath9k_htc.

With the sc gone we can now just pass the common structure to
ath_rx_prepare().

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