ath9k: Fix mactime from being clobbered in rx_status

mactime was being overwritten by the function ath9k_rx_skb_preprocess. Fixed by
memsetting rx_status in ath_rx_tasklet.

Signed-off-by: Ashok Nagarajan <ashok@cozybit.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>