wifi: mac80211_hwsim: send a beacon per link

Add to hwsim a link_data structure.
For now, move the beacon interval and beacon timer to
the link_data structure (making them per link).
Set a beacon timer for each link and transmit a per
link beacon (pass the link_id to ieee80211_beacon_get).

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>