AP: Enable WMM with default parameters by default for HT

AP: Enable WMM with default parameters by default for HT

If WMM is not disabled explicitly (wmm_enabled=0 in hostapd.conf),
enable WMM automatically whenever HT (ieee80211n) is enabled. Use
the default WMM parameters for AP TX queues and the EDCA parameters
advertised for stations in WMM IE if no overriding values are
included in the configuration.