cfg80211: add support to configure HE MCS for beacon rate

This allows an option to configure a single HE MCS beacon tx rate.

Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
Link: https://lore.kernel.org/r/1602879327-29488-2-git-send-email-rmanohar@codeaurora.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>