iwlwifi: mvm: report TX rate to mac80211 directly for RS offload

If we have offloaded rate scaling, which is always true for those
devices supporting HE, then report the TX rate directly from the
data the firmware gives us, instead of only passing it to mac80211
on frame status only and for it to track it.

First of all, this makes us always report the last good rate that
the rate scaling algorithm picked, which is better than reporting
the last rate for any frame since management frames etc. are sent
with very low rates and could interfere.

Additionally, this allows us to properly report HE rates, though
in case there's a lot of trigger-based traffic, we don't get any
choice in the rates and don't report that properly right now.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>