netifd: fix mac80211.sh to respect setting of mcast_rate

This patch fixes a bug in /lib/netifd/wireless/mac80211.sh, where
the UCI setting of wireless multicast traffic (in uci: mcast_rate)
is not respected within netifd. Especially in Freifunk mesh networks
the olsr routing as effected by this, as only the lowest mcast_rate
was used, even when uci ncast_rate was set to something else.
In function mac80211_setup_adhoc() the value of mcast_rate is missing
in json_get_vars.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>

SVN-Revision: 39232