I'm not sure where in the deep, distant past I found the AR_PHY_MODE
registers for half/quarter rate mode, but unfortunately that doesn't
seem to work "right" for non-AR9280 chips.
Specifically:
* don't touch AR_PHY_MODE
* set the PLL bits when configuring half/quarter rate
I've verified this on the AR9280 (5ghz fast clock) and the AR5416.
The AR9280 works in both half/quarter rate; the AR5416 unfortunately
only currently works at half rate. It fails to calibrate on quarter rate.