ath9k_hw: Fix power on reset

Commit "ath9k_hw: add an extra delay when reseting AR_RTC_RESET"
added an extra udelay to the reset routine. As the required
delay is already present, remove this.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>