iwlwifi: mvm: remove PS from lower rates.

Power save (PS) should only be enabled when we reach the max phy rate.
Before we reach it (MCS_9) for VHT, we should keep trying to improve the
throughput.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210411124417.2a2fb9f9c25e.I7c7bbcfbdc1d35d2c3338778fb397dd5b08ea0e8@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>