wifi: iwlwifi: mvm: implement link grading

For selecting what link(s) out of the usable ones
to activate, calculate a grade for a given link.

Calculation of a link grade is done as follows:
1. get the estimated throughput according to the RSSI of the link, this
   will be the base grade
2. get the channel load from the BSS Load Element, subtracting the load
   caused by us. Apply the factor on the grade.
3. puncturing factor: calculate the percentage of the punctured
   subchannels (out of the total subchannels). Apply this on the grade.

The link grading will be used by the link selection mechanism in a later
patch.

Also add KUnit tests for it.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Link: https://msgid.link/20240416134215.a6799dbd5643.If137ca6dc443606c7d8c99ec1fc38b325003a7c1@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>