ath9k: Choose correct ANI calibration period

ANI can't be turned on/off dynamically yet, but the calculation
of the calibration period is wrong anyway. This patch fixes it.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>