mt76: add support for setting mcast rate

Mesh and ad-hoc interfaces allow users to set mcast rate through
NL80211_ATTR_MCAST_RATE, so rework mt76_default_basic_rate() to
adapt that.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>