net80211: check if beacon contains 11b rates when ERP IE is absent before enabling protection.

[avos]

Summary

Accordingly to IEEE 802.11-2012, 10.15.8:
"... if the DSSS/CCK Mode in 40 MHz subfield is equal to 0, then the
following apply:
...

• The AP shall not include an ERP element in its Beacon and Probe Response

frames.

• 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?

[adrian]

Hm, have you tested this against 11n sta/ap's ?

[avos]

That was a bit problematic:

• there are no 40MHz-capable APs without DSSS/CCK support in the range;
• FreeBSD / Linux inserts ERP/11b rates into beacon even when DSSS/CCK support is not advertised.

To ensure that this will work as expected I've added some checks into ieee80211_construct_beacon(); result:

• no is11bclient() check - second rtwn AP turns protection on;
• with is11bclient() check - net80211 sees that 40MHz AP supports non-11b rates and

keeps protection off.