Third set of iwlwifi patches for 4.20
* Fix for a race condition that caused the FW to crash;
* HE radiotap cleanup and improvements;
* Reorder channel optimization for scans;
* Bumped the FW API version supported after the last API change for
this release;
* Debugging improvements;
* A few bug fixes;
* Some cleanups in preparation for a new implementation;
* Other small improvements, cleanups and fixes.