iwlwifi: mvm: rs: dynamically switch between 80MHz and 20MHz in some scenarios

This is a tweak which has been shown to improve performance when
moving away from the AP while working in 80Mhz.
When RS decides to go down to 80MHz SISO MCS0 instead switch to 20MHz MCS4.
Go back to 80MHz MCS1 if RS can sustain 20MHz MCS5.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>