wifi: iwlwifi: mld: skip unknown FW channel load values

The firmware statistics were previously reporting bogus/old
channel load values if the device hadn't been active on a
given channel; it'll report an unknown value now for those
statistics affected (channel_load and channel_load_not_by_us.)
Handle that by simply skipping the value, the averaging would
result in the exact same value as before.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250424153620.db5410318642.I4d2981f68b915ad335bb02c926e9289c2a60ea6c@changeid


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