iw: fix HT PHY BSS Membership selector value encoding

iw: fix HT PHY BSS Membership selector value encoding

IEEE 802.11n 7.3.2.2 extended the supported rate IE to
support a special encoding for HT rate support.

iw needs to be updated in order to recognize
the magic value and parse it accordingly.

e.g.:

> Extended supported rates: 63.5*

now becomes:

> Extended supported rates: HT*

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>