can: Add SPDX license identifiers for CAN subsystem

Add missing SPDX identifiers for the CAN network layer and correct the SPDX
license for two of its include files to make sure the BSD-3-Clause applies
for the entire subsystem.
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

include/linux/can/core.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause) */
 /*
  * linux/can/core.h
  *

include/linux/can/skb.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause) */
 /*
  * linux/can/skb.h
  *

net/can/af_can.c

+// SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause)
 /*
  * af_can.c - Protocol family CAN core module
  *            (used by different CAN protocol modules)

net/can/af_can.h

+/* SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause) */
 /*
  * Copyright (c) 2002-2007 Volkswagen Group Electronic Research
  * All rights reserved.

net/can/bcm.c

+// SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)
 /*
  * bcm.c - Broadcast Manager to filter/send (cyclic) CAN content
  *

net/can/gw.c

+// SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)
 /*
  * gw.c - CAN frame Gateway/Router/Bridge with netlink interface
  *

net/can/proc.c

+// SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)
 /*
  * proc.c - procfs support for Protocol family CAN core module
  *

net/can/raw.c

+// SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)
 /*
  * raw.c - Raw sockets for protocol family CAN
  *