mac80211: Add HT operation modes for IBSS

The HT mode is set by iw (previous patchsets).
The interface is set into the specified HT mode.
HT mode and capabilities are announced in beacons.

If we add a station that uses HT also, the fastest matching HT mode will
be used for transmission. That means if we are using HT40+ and we add a station
running on HT40-, we would transfer at HT20.

If we join an IBSS with HT40, but the secondary channel is not
available, we will fall back into HT20 as well.

Allow frame aggregation to start in IBSS mode.

Signed-off-by: Alexander Simon <an.alexsimon@googlemail.com>
[siwu@hrz.tu-chemnitz.de: Updates]
* remove implicit channel_type enum assumptions
* use rate_control_rate_init() if channel type changed
* remove channel flags check
* activate HT IBSS feature support
* slightly reword commit message
* rebase on wireless-testing

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>