mt76: mt7915: fix unbounded shift in mt7915_mcu_beacon_mbss

Fix the following smatch static checker warning:
	drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:1872 mt7915_mcu_beacon_mbss()
	error: undefined (user controlled) shift '(((1))) << (data[2])'

Rely on mac80211 definitions for ieee80211_bssid_index subelement.

Fixes: 6b7f9aff

 ("mt76: mt7915: introduce 802.11ax multi-bss support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>