ath9k_hw: fix PLL clock initialization for newer SoC

On AR934x and newer SoC devices, the layout of the AR_RTC_PLL_CONTROL
register changed. This currently breaks at least 5/10 MHz operation.
AR933x uses the old layout.

It might also have been causing other stability issues because of the
different location of the PLL_BYPASS bit which needs to be set during
PLL clock initialization.

This patch also removes more instances of hardcoded register values in
favor of properly computed ones with the PLL_BYPASS bit added.

Reported-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>