Bluetooth: fix uninitialized variables notify_evt

Coverity Scan report:

[...]
*** CID 1493985:  Uninitialized variables  (UNINIT)
/net/bluetooth/hci_event.c: 4535 in hci_sync_conn_complete_evt()
4529
4530     	/* Notify only in case of SCO over HCI transport data path which
4531     	 * is zero and non-zero value shall be non-HCI transport data path
4532     	 */
4533     	if (conn->codec.data_path == 0) {
4534     		if (hdev->notify)
>>>     CID 1493985:  Uninitialized variables  (UNINIT)
>>>     Using uninitialized value "notify_evt" when calling "*hdev->notify".
4535     			hdev->notify(hdev, notify_evt);
4536     	}
4537
4538     	hci_connect_cfm(conn, ev->status);
4539     	if (ev->status)
4540     		hci_conn_del(conn);
[...]

Although only btusb uses air_mode, and he only handles HCI_NOTIFY_ENABLE_SCO_CVSD
and HCI_NOTIFY_ENABLE_SCO_TRANSP, there is still a very small chance that
ev->air_mode is not equal to 0x2 and 0x3, but notify_evt is initialized to
HCI_NOTIFY_ENABLE_SCO_CVSD or HCI_NOTIFY_ENABLE_SCO_TRANSP. the context is
maybe not correct.

Let us directly use the required function instead of re-initializing it,
so as to restore the original logic and make the code more correct.

Addresses-Coverity: ("Uninitialized variables")
Fixes: f4f9fa0c ("Bluetooth: Allow usb to auto-suspend when SCO use	non-HCI transport")
Suggested-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>