Bluetooth: hci_qca: fix NULL pointer dereference in qca_dmp_hdr() for non-serdev device

hu->serdev is NULL for hci_uart attached via non-serdev paths, but
qca_dmp_hdr() unconditionally dereferences hu->serdev->dev.driver->name,
causing a NULL pointer dereference.

Fix by guarding the dereference with a NULL check and falling back to
"hci_ldisc_qca" for the non-serdev case.

Fixes: 06d3fdfcdf ("Bluetooth: hci_qca: Add qcom devcoredump support")
Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

drivers/bluetooth/hci_qca.c

@@ -1028,7 +1028,7 @@ static void qca_dmp_hdr(struct hci_dev *hdev, struct sk_buff *skb)
 	skb_put_data(skb, buf, strlen(buf));
 
 	snprintf(buf, sizeof(buf), "Driver: %s\n",
-		hu->serdev->dev.driver->name);
+		 hu->serdev ? hu->serdev->dev.driver->name : "hci_ldisc_qca");
 	skb_put_data(skb, buf, strlen(buf));
 }