cfg80211: always set IBSS basic rates

IBSS started from wireless extensions is currently
missing basic rate configuration, fix this by moving
the code to generate the default to the common code
that gets invoked for both nl80211 and wext.

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