The function adding the supported channels IE during a TDLS connection had
several issues:
1. If the entire subband is usable, the function exitted the loop without
adding it
2. The function only checked chandef_usable, ignoring flags like RADAR
which would prevent TDLS off-channel communcation.
3. HT20 was explicitly required in the chandef, while not a requirement
for TDLS off-channel.