If no mcast_rate is set for the wifi-iface then there is no rate_idx (0)
set for the bss. This breaks for example 5GHz meshpoint interfaces because
0 maps to a CCK rate (11Mbit/s).
Fixes: db90c243a0b9 ("mac80211: update to version based on 4.19-rc4")
Signed-off-by: Sven Eckelmann <sven@narfation.org>