mt76: fix MBSS index condition in DBDC mode

MT7915_MAX_INTERFACES is per-band declaration in MT7915/MT7986/MT7916.
Enlarge vif_mask to 64 bits wide, including the bit operation.

Reviewed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Bo Jiao <bo.jiao@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>