mac80211: always advertise STBC/MCSes even if no AP support

Advertise STBC capabilities and MCS rates even if the AP
doesn't support them. This has always been the right thing
to do, but used to be problematic with some APs. Now WFA
testing requires this so re-enable it, problematic APs
would then presumably not pass the test and be fixed.

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