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.
The v1 and v3 interfaces returned the whole EEPROM but the v14/v4k
interfaces just returned the base header. There's extra information
outside of that which would also be nice to get access to.
