spi: bcm53xx: (re)license code to the GPL v2

My intention was to release this code under GPL v2 license. For some
reason my initial commit 0fc6a323 ("spi: bcm53xx: driver for SPI
controller on Broadcom bcma SoC") totally missed licensing info.
MODULE_LICENSE was later added by Axel specifying "GNU Public License
v2 or later".

This patch clarifies situation by adding a proper header (with Copyright
line) and adjusting MODULE_LICENSE. It should be acked by every driver
contributor.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Jingoo Han <jingoohan1@gmail.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Acked-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Vaishali Thakkar <vaishali.thakkar@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm53xx.c

+/*
+ * Copyright (C) 2014-2016 Rafał Miłecki <rafal@milecki.pl>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
 #define pr_fmt(fmt)		KBUILD_MODNAME ": " fmt
 
 #include <linux/kernel.h>
@@ -355,4 +363,4 @@ module_exit(bcm53xxspi_module_exit);
 
 MODULE_DESCRIPTION("Broadcom BCM53xx SPI Controller driver");
 MODULE_AUTHOR("Rafał Miłecki <zajec5@gmail.com>");
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");