genirq/matrix: Cleanup SPDX identifier

Use the proper SPDX-Identifier format.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Link: https://lkml.kernel.org/r/20180314212030.492674761@linutronix.de

kernel/irq/matrix.c

@@ -1,8 +1,6 @@
-/*
- * Copyright (C) 2017 Thomas Gleixner <tglx@linutronix.de>
- *
- * SPDX-License-Identifier: GPL-2.0
- */
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2017 Thomas Gleixner <tglx@linutronix.de>
+
 #include <linux/spinlock.h>
 #include <linux/seq_file.h>
 #include <linux/bitmap.h>