iwlwifi: mvm: rs: disable MCS9 Tx workaround

MCS9 introduces some corner cases in the current rs
algorithm which may lead to non optimal throughput and
instability in the throughput. Until all the corner
cases are resolved disable MCS9 for Tx as a workaround
which yields better throughput results as MCS8 is much
more stable.

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