spi: Fix formatting of header block

Make everything look intentional by having a C++ comment for the whole
block, not just the SPDX line.
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

 // SPDX-License-Identifier: GPL-2.0-or-later
-/*
- * SPI init/core code
- *
- * Copyright (C) 2005 David Brownell
- * Copyright (C) 2008 Secret Lab Technologies Ltd.
- */
+// SPI init/core code
+//
+// Copyright (C) 2005 David Brownell
+// Copyright (C) 2008 Secret Lab Technologies Ltd.
 
 #include <linux/kernel.h>
 #include <linux/device.h>