mac80211: set only VO as a U-APSD enabled AC

Some APs experience problems when working with U-APSD. Decrease the
probability of that happening by using legacy mode for all ACs but VO.

The AP that caused us troubles was a Cisco 4410N. It ignores our
setting, and always treats non-VO ACs as legacy.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>