ath9k_hw: fix clock rate calculations for ANI

The clock rate was assumed to be static but it actually
changes depending on the mode of operation, correct this
to help improve the calcuation of the listenTime for ANI.
This change will help adjust ANI more accurately on different
PHY thresholds.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>