staging: pi433: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header files related to pi433 radio module drivers.
It assigns explicit block comment to the SPDX License Identifier.

Changes made by using a script provided by Joe Perches here:
https://lkml.org/lkml/2019/2/7/46.
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Link: https://lore.kernel.org/r/20200208083621.GA3797@nishadSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.h

-/* SPDX-License-Identifier: GPL-2.0+
- *
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
  * include/linux/TODO
  *
  * userspace interface for pi433 radio module

drivers/staging/pi433/rf69.h

-/* SPDX-License-Identifier: GPL-2.0+
- *
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
  * hardware abstraction/register access for HopeRf rf69 radio module
  *
  * Copyright (C) 2016 Wolf-Entwicklungen

drivers/staging/pi433/rf69_enum.h

-/* SPDX-License-Identifier: GPL-2.0+
- *
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
  * enumerations for HopeRf rf69 radio module
  *
  * Copyright (C) 2016 Wolf-Entwicklungen

drivers/staging/pi433/rf69_registers.h

-/* SPDX-License-Identifier: GPL-2.0+
- *
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
  * register description for HopeRf rf69 radio module
  *
  * Copyright (C) 2016 Wolf-Entwicklungen