mac80211: Use suitable semantics for beacon availability indication

Currently beacon availability upon association is marked by have_beacon
flag of assoc_data structure that becomes unavailable when association
completes. However beacon availability indication is required also after
association to inform a driver. Currently dtim_period parameter is used
for this purpose. Move have_beacon flag to another structure, persistant
throughout a interface's life cycle. Use suitable sematics for beacon
availability indication.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
[fix another instance of BSS_CHANGED_DTIM_PERIOD in docs]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>