ath9k_hw: fix regression in ANI listen time calculation

  wireless-testing
  commit 37e5bf65
  Author: Luis R. Rodriguez <lrodriguez@atheros.com>
  Date:   Sat Jun 12 00:33:40 2010 -0400

    ath9k_hw: fix clock rate calculations for ANI

This commit accidentally broke clock rate calculation by doubling the
calculated clock rate

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>