mac80211: eliminate IBSS warning in rate_lowest_index()

In IBSS mode prior to join/creation of new IBSS it is possible that
a frame from unknown station is received and an ibss_add_sta() is
called. This will cause a warning in rate_lowest_index() since the
list of supported rates of our station is not initialized yet.

The fix is to add ibss stations with a rate we received that frame
at; this single-element set will be extended later based on beacon
data. Also there is no need to store stations from a foreign IBSS.

Signed-off-by: Vladimir Koutny <vlado@ksp.sk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>