ath9k: add duration field to ath_tx_status

Add duration field to ath_tx_status in order to report frame duration for each
entry in multi-retry chain. These fields will be used in ACK timeout estimation
algorithm (dynack)

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>