ath9k_hw: add a private callback for PLL control computation

The PLL control computation used to program the AR_RTC_PLL_CONTROL
register varies between our harware so just add a private callback for it.
AR9003 will use its own callback.

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