ath9k: dyanck: introduce ath_dynack_set_timeout routine

Introduce ath_dynack_set_timeout routine to configure slottime/ack/cts
timeouts and remove duplicated code

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