ath9k: dynack: set ackto to max timeout in ath_dynack_reset

Initialize acktimeout to the maximum configurable value in
ath_dynack_reset in order to not disconnect long distance static links
enabling dynack and even to take care of possible errors configuring
a static timeout. Moreover initialize station timeout value to the current
acktimeout value

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