mac80211: add HE 6 GHz capability only if supported

The HE 6 GHz capability should only be included if there
are actually available channels on 6 GHz, and if that's
the case we need to get it from the 6 GHz band data, not
whatever other band we're on now.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.290bf5c87030.I178aff1c3a6e32456d4ac9238e4a2eb47d209ccd@changeid
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.05e935e8dd98.I83ff7eb2ae8ebdf2e30c4fa2461344d9e569f599@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>