On WiNG 5.5, TKIP only encryption is not available. However, if this was configured on a previous version prior to an upgrade, this configuration will be preserved. If it is desired to configure TKIP-CCMP, it is needed to make this configuration change from the CLI, with the command no service wpa-wpa2 exclude-ccmp
If CCMP is enabled, this may be due to WMM being disabled. See WLANs are not advertising 802.11n data rates, despite being configured for 802.11a/n.