iwlwifi: mvm: Direct multicast frames to the correct station

Multicast frames for NL80211_IFTYPE_AP and NL80211_IFTYPE_ADHOC were
directed to the broadcast station, however, as the broadcast station
did not have keys configured, these frames were sent unencrypted.

Fix this by using the multicast station which is the station for which
encryption keys are configured.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>