ath9k: do not pass the entire descriptor to ath_rx_prepare()

Its not needed, so just pass the hardware RX status.
We'll be simplfying ath_rx_prepare() with code we can share
between ath9k and ath9k_htc. This will help make that code
easier to read and manage.

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