mt76: mt7615: fix event report in mt7615_mcu_bss_event

Currently mt7663 devices do not support DBDC so fw events have no info
about it. Fix mt7615_mcu_bss_event that wrongly use bss_idx as DBDC
band_idx while it is vif index.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>