mac80211: split off ibss disconnect

IBSS CSA will require to disconnect if a channel switch fails, but
mac80211 should search and re-connect after this disconnect. To allow
such usage, split off the ibss disconnect process in a separate function
which only performs the disconnect without overwriting nl80211-supplied
parameters.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>