ath11k: clear DCM max constellation tx value

According to 11ax spec. draft 4.0. DCM Max Constellation Tx data
field should be set to "Reserved" for an AP, therefore bit is
cleared.

Signed-off-by: Muna Sinada <msinada@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1590707417-29672-1-git-send-email-msinada@codeaurora.org