ath9k: Fix RTC_DERIVED_CLK usage

Based on the reference clock, which could be 25MHz or 40MHz,
AR_RTC_DERIVED_CLK is programmed differently for AR9340 and AR9550.
But, when a chip reset is done, processing the initvals
sets the register back to the default value.

Fix this by moving the code in ath9k_hw_init_pll() to
ar9003_hw_override_ini(). Also, do this override for AR9531.

Cc: stable@vger.kernel.org
Signed-off-by: Miaoqing Pan <miaoqing@qca.qualcomm.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>