iwlwifi: mvm: rs: add Tx STBC support

STBC is enabled on HT/VHT SISO rates in case we don't care
about power consumption and it won't hurt BT.
This is done only in case the peer and our chip support STBC
of course.
While at it fixed a bug which was causing bw and ldpc
flags to be set incorrectly in the rate scale table in case
we are switching to a legacy Tx column. This had no real impact.

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