ath9k: use timespec64 for tsf_ts

ath9k is the last remaining user of the deprecated getrawmonotonic()
interface. There is nothing wrong with this usage, but migrating
to a timespec64 based interface lets us clean up the old API.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>