ath9k: dynack: properly set last timeout timestamp in ath_dynack_reset

Add compute timeout to last computation timestamp in
ath_dynack_reset in order to not run ath_dynack_compute_ackto
immediately

Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>