Add support for discovery of hidden networks on the 6GHz band,
by including the scan request direct SSIDs in the FW scan request
command:

- In case a short SSID matches one of the direct SSIDs in the scan
  request command, add the matching SSID in the same offset in the
  'direct_ssids' array.
- Otherwise, add the SSID in one of the available slots.

Additionally, as a preparation to handle hidden APs, refactor
iwl_mvm_umac_scan_cfg_channels_v6_6g() the function.
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210802170640.ffb540a70212.Ia2bb9bc9435b833820bcc7dc30adcedb5a5a9869@changeidSigned-off-by: Luca Coelho <luciano.coelho@intel.com>