mac80211: IBSS supported rate fixes

Currently mac80211 announces a rate set with no basic rates,
this fixes it to use 1/2 or 6/9 Mbit as basic rates by default.
Additionally, mac80211 will currently adopt the peer's entire
rate set, rather than just the basic rate set; fix that too.

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