hostapd: dfs: Limitation of VHT80 channels

[Sven Eckelmann]

Hi,

we were asking ourselves why you've introduced in 58b73e3dd96c ("hostapd: DFS 
with 40/80 MHz channel width support"), 1dc17db34ac4 ("DFS: Fix available 
channels list for VHT80") and  4f1e01b8e3ff ("DFS: Add VHT160 available 
channels") special limitations for CSA target channels.

The example from Daniel Bailey was a US VHT80 configuration (ath10k) which 
received a relevant radar signal pattern and now has to switch the channel. 
The channel list contained a the non-DFS channel 44 and he expected that the 
device switches to this channel. But instead, the channel was only marked as 
unusable and hostapd did nothing (aka: was still sending at the now "unusable" 
channel range). Not the best idea when you want to get a DFS certified AP.

This could be worked around by replacing channel 44 with channel 36. But it 
would be good to understand why VHT80 channels were limited to 36, 52, 100, 
116, 132, 149

Kind regards,
	Sven
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