When the fast clock (44mhz) is enabled for 5ghz HT20, the
dual ADCs aren't enabled. Trying to do the ADC calibrations
here would result in calibration never completing; this
resulted in IQ calibration never running and thus performance
issues in 11a/11n HT20 mode.
Leave it enabled for non-fastclock (40mhz) 11a mode and
HT40 modes.
This has been fixed in discussion with Felix Fietkau (nbd)
and discussions with the Atheros baseband team.
Linux ath9k now has a similar fix.
Approved by: re (kib)