hostapd: fix mcast_rate setting

Introduced by ("netifd: add wireless configuration support and port mac80211 to
the new framework")

Reported-by: René van Weert <r.vanweert@sowifi.com>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39288 3c298f89-4303-0410-b956-a3cf2f4a3e73