staging: comedi: drivers: Cleanup malformed license identifier

This patch fixes the checkpatch.pl warning:
WARNING: Missing or malformed SPDX-License-Identifier tag in line 1

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Stephen Davies <davo2002@tpg.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/8255.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * module/8255.h
  * Header file for 8255