mac80211: move ibss presp generation in own function

Channel Switch will later require to generate beacons without setting
them immediately. Therefore split the presp generation in an own
function. Splitting the original very long function might be a good idea
anyway.

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>