mac80211: refactor common auth/assoc setup code

As associating is possible without first authenticating
(for FT over DS) association also has to be able to
switch to the right channel, insert the station entry
etc. Factor out this common code into a new function
called ieee80211_prep_connection().

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