ath9k: dynack: remove 'experimental' tag

Remove experimental tag from dynack Kconfig entry since it has
been tested on outdoor 25Km links

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