nl80211: fix beacon tx rate mask validation

While adding HE MCS beacon tx rate support, it is observed that legacy
beacon tx rate in VHT hwsim test suite is failed. Whenever the
application doesn't explicitly set VHT/MCS rate attribute in fixed rate
command, by default all HE MCS masks are enabled in cfg80211. In beacon
fixed rate, more than one rate mask is not allowed. Fix that by not
setting all rate mask by default in case of beacon tx rate.

Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
Link: https://lore.kernel.org/r/1602879327-29488-1-git-send-email-rmanohar@codeaurora.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>