mac80211: use min rate as basic rate for buggy APs

Some buggy APs (and even P2P_GO) don't advertise their
basic rates in the association response.

In such case, use the min supported rate as the
basic rate.

Reported-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>