mac80211: implement set_mcast_rate() callback

This new callback can be used to tune the rate to be used to send
multicast frames.

In the current state the multicast rate can be specified on IBSS/MESH
joining only. This makes it impossible to select a custom multicast
rate when then join command is sent by an external program (e.g.
wpa_supplicant)

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