ar9170: support HT receive and channel config

This patch adds support for configuring HT40 channels
and receiving HT40 to ar9170. Receiving aggregation
doesn't seem to work right now, so it's not enabled.
Same goes for TX aggregation, but that probably needs
even more work.

With this, I can receive roughly 33 Mbits/sec.

The HT capabilities are a little odd, I tried following
otus here -- in particular having SGI_40 but not SGI_20
is a little weird but afaict that's what otus does.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>