ath9k: Fix TSF processing

There is no need to calculate the mactime for chained
descriptor packets, so make sure that this is done
only for the last fragment of valid packets.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>