hostapd: set mcast_rate in mesh mode

The wpa_supplicant code for IBSS allows to set the mcast rate. It is
recommended to increase this value from 1 or 6 Mbit/s to something higher
when using a mesh protocol on top which uses the multicast packet loss as
indicator for the link quality.

This setting was unfortunately not applied for mesh mode. But it would be
beneficial when wpa_supplicant would behave similar to IBSS mode and set
this argument during mesh join like authsae already does. At least it is
helpful for companies/projects which are currently switching to 802.11s
(without mesh_fwding and with mesh_ttl set to 1) as replacement for IBSS
because newer drivers seem to support 802.11s but not IBSS anymore.

Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Tested-by: Simon Wunderlich <simon.wunderlich@openmesh.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [refresh]