iwlwifi: bring in some of the rs code

Bring in code to call into rs-fw.c functionality for firmware
supported rate select.

Anything before AX200 (firmware matching iwlwiif-[0-9]*.ucode) will
still need further implementation, and so does iwl_mvm_rs_tx_status().
These bits marked with compat.linuxkpi.80211.debug TODO()s for now.

Implement some lindebugfs bits to gather more statistical information.

Sponsored by: The FreeBSD Foundation
Obtained from: bz_wifi_11n branch (partially)
MFC after: 3 days