Converting from freq to channel only needed the 6 GHz freq. range
to be added, however, converting from channel to freq. will require
the function ieee80211_chan_to_mhz to take a starting frequency as
there's overlap in the channel numbering between 2.4/5 GHz and 6 GHz
bands. This may not be possible in some cases, so for now the
function will continue to do the conversion based on the order
on which the freq. ranges are defined. Specifically, it will favor
2.4/5 GHz over 6 GHz.