mt76: mt7615: fix potential overflow on large shift

Fix the following static checker warning:
error: undefined (user controlled) shift '(((1))) << (c->omac_idx)'

Fixes: 402a695b ("mt76: mt7615: fix CSA notification for DBDC")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/mcu.c

@@ -411,6 +411,9 @@ mt7615_mcu_rx_csa_notify(struct mt7615_dev *dev, struct sk_buff *skb)
 
 	c = (struct mt7615_mcu_csa_notify *)skb->data;
 
+	if (c->omac_idx > EXT_BSSID_MAX)
+		return;
+
 	if (ext_phy && ext_phy->omac_mask & BIT_ULL(c->omac_idx))
 		mphy = dev->mt76.phy2;