iwlwifi: mvm: Add FTM initiator RTT smoothing logic

iwlwifi: mvm: Add FTM initiator RTT smoothing logic

The overcome instabilities in the RTT results add smoothing logic
to the reported results. In short, the smoothing logic tracks the
RTT average of each responder for a period of time, and in case
a new RTT results is found to be a spur, the tracked RTT average
is reported instead of the current RTT measurement.

Smooth logic debug configuration using iwl-dbg-cfg.ini:

- MVM_FTM_INITIATOR_ENABLE_SMOOTH: Set to 1 to enable smoothing logic
 (default=0).
- MVM_FTM_INITIATOR_SMOOTH_ALPHA: A value between 0 - 100, defining
  the weight of the current RTT results vs. the RTT average tracked
  based on the previous results. A value of 100 means use only the
  current RTT results.
- MVM_FTM_INITIATOR_SMOOTH_AGE_SEC: The maximal time in seconds in which
  the RTT average tracked based on previous results is considered valid.
- MVM_FTM_INITIATOR_SMOOTH_UNDERSHOOT: if the current RTT is positive
  and below the RTT average by at least this value, report the average
  RTT instead of the current one. In units of picoseconds.
- MVM_FTM_INITIATOR_SMOOTH_OVERSHOOT: if the current RTT is positive
  and above the RTT average by at least this value, report the average
  RTT instead of the current one. In units of picoseconds.

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.20200930161256.48a9cec2081b.Iaec1e29f738232adfe9e2ea8e9eb9b6ff0323ae1@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>