mac80211: handle HT PHY BSS membership selector value correctly

802.11n-2009 extends the supported rates element with a
magic value which can be used to prevent legacy stations
from joining the BSS.

However, this magic value is not a rate like the others
and the magic can simply be ignored/skipped at this late
stage.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com&gt;--->
Signed-off-by: John W. Linville <linville@tuxdriver.com>