[net80211] announce 11n capabilities in probe requests in IBSS mode.

The 802.11-2012 specification notes that a subset of IEs should be present
in IBSS probe requests. This is what (initially) allows nodes to discover
that other nodes are 11n capable. Notably - HTCAP, but not HTINFO.

This isn't everything required to reliably enable 11n between net80211
peers; there's more work to come.

Tested:

• AR9380, IBSS+11n mode