Solve txpower spikes during minstrel(_ht) sampling with only fixed_txpower

[ecsv]

It was noticed that my high power ath9k device had sudden txpower burst which completely ruined the transmission to my remote device because its ADC was completely saturated. This resulted in rather low throughput because these high power packets were minstrel(_ht) sample packets. So the device never reached its expected high MCS rates.