This is something bus clock related from what I can gather. It is needed for
the AR9220 based Ubiquiti SR71-12 and SR71-15 Mini-PCI NICs.
(Note: those NICs don't work right now because of earlier changes to handle
power table offset correctly. That'll be resolved in a follow-up commit.)
It defaults to -5 dBm for eeproms earlier than v21.
This apparently only applies to Merlin (AR9280) or later,
earlier 11n chipsets have a power table offset of 0.
All the code in ath9k which checks the power table offset
and takes it into account first ensures the chip is
Merlin or later.
