nl80211: add support for setting fixed HE rate/gi/ltf

This patch adds the nl80211 structs, definitions, policies and parsing
code required to pass fixed HE rate, GI and LTF settings.

Signed-off-by: Miles Hu <milehu@codeaurora.org>
Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20200804081630.2013619-1-john@phrozen.org
[fix comment]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>