iwlwifi: mvm: rs: refactor to allow direct rs updating

Enable RS to get updated directly via iwl_mvm_rs_tx_status
which will be called by the driver and not via mac80211
rate control. This is required for a following patch to
report on BAs which do not cause the BA window to progress.
Also this is a first step in decoupling rs from the
mac80211 rate control.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>