Staging: unisys: Correct malformed SPDX-License-Identifier

Correct SPDX-License-Identifier comment characters to silence
checkpatch.pl warning:
WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
Signed-off-by: Wentao Cai <etsai042@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/unisys/include/iochannel.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Copyright (C) 2010 - 2016 UNISYS CORPORATION
  * All rights reserved.