wifi: iwlwifi: handle per-phy statistics from fw

In the operational statistics notifications (both old and new
API) the driver receives the statistics per phy.
currently this statistics wasn't handled because they wasn't needed.

Now the channel_load_by_us parameter in these statistics will be used
for the link grading calculation (implemented in another patch),
so store its value in phy_ctxt.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Link: https://msgid.link/20240218194912.e84f975b69ee.Ibbc7817135827e45adaaa47b796be165f9f1ca48@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>