mt76: mt7915: fix muar_idx in mt7915_mcu_alloc_sta_req()

[ Upstream commit 161cc139 ]

For broadcast/multicast wcid, the muar_idx should be 0xe.

Fixes: e57b7901 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>