wifi: iwlwifi: mvm: refactor scan channel description a bit

wifi: iwlwifi: mvm: refactor scan channel description a bit

The channel number is at the same position across all versions
of the channel description struct, so move it out of the union
that versions it. Also add __packed annotations to all of the
sub-structs and the union so it's packed correctly, and fully
document the structure.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Daniel Gabay <daniel.gabay@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20240901071542.e31623ae4201.I1ea69a8ec3d39492f39d84e31fb105b159359c28@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>