o only include 1/2 and 1/4 width channels when they are specified in the
regulatory database description; previously we treated them as if they
were part of the band and blindly added them for 11a/g
o check the channel list returned in the devcaps to identify whether a
device supports 1/2 or 1/4 width channels on a band; this might be
better brought out as a capability bit to avoid filling the channel
list w/ 1/2 and 1/4 width channels but then cards that only support
these channels in a range of frequencies could not be described (though
right now we don't check frequency range only band)
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