staging: pi433: add SPDX-License-Identifier tag

Use GPL-2.0+ based on the license text in each of the files.
Signed-off-by: Valentin Vidic <Valentin.Vidic@CARNet.hr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

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

drivers/staging/pi433/pi433_if.h

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

drivers/staging/pi433/rf69.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * abstraction of the spi interface of HopeRf rf69 radio module
  *

drivers/staging/pi433/rf69.h

-/*
+/* 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+
+ *
  * enumerations for HopeRf rf69 radio module
  *
  * Copyright (C) 2016 Wolf-Entwicklungen

drivers/staging/pi433/rf69_registers.h

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