ath9k: dynack: set max timeout according to channel width

Compute maximum configurable ackimeout/ctstimeout according to channel
width (clockrate)

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