The HT40 channel population logic was "just" doing pairs of channels starting with
the band entry frequency. Trouble is, a lot of the rules start way off at 5120MHz,
which isn't a valid 5GHz channel. Then, eg for HT40U, it would populate:
* (5120,5140)
* (5160,5180)
* (5200,5220)
* (5240,5260)
.. as the HT40U pairs, with the first being the primary channel. Channel 36
is 5180MHz, and since it's not a primary channel here, it wouldn't populate it.
Then, the next HT40U would be 5200/5220, which is highly wrong.
HT40D had the same problem.
So, this just forces that 5GHz HT40 channels start at channel 36 (5180),
no matter what the band edge says. This includes eg doing 4.9GHz channels.
This erm, meant that the HT40 channels for the low band was always wrong.
Oops!
Tested:
* AR9380, STA mode
* AR9344 SoC, AP mode
MFC after: 1 week
eee8e3627b