Accordingly to IEEE 802.11-2012, 10.15.8:
"... if the DSSS/CCK Mode in 40 MHz subfield is equal to 0, then the
- The AP shall not include an ERP element in its Beacon and Probe Response
- The AP shall not include DSSS/CCK rates in the Supported Rates element."
P.S. should we check other bits (htcap & DSSS/CCK, htinfo & 40 MHz) too?