nl/cfg80211: add the NL80211_CMD_SET_MCAST_RATE command

This command triggers a new callback: set_mcast_rate(). It enables
the user to change the rate used to send multicast frames for vif
configured as IBSS or MESH_POINT

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>