wifi: rt2x00: set SoC wmac clock register

Instead of using the default value 33 (pci), set US_CYC_CNT init based
on Programming guide:
If available, set chipset bus clock with fallback to cpu clock/3.

Reported-by: Serge Vasilugin <vasilugin@yandex.ru>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/3e275d259f476f597dab91a9c395015ef3fe3284.1663445157.git.daniel@makrotopia.org