staging: comedi: comedidev.h: Fix SPDX-License-Identifier tag style

Replace // SPDX-License-Identifier: GPL-2.0+ by
/* SPDX-License-Identifier: GPL-2.0+ */ as per licensing rule
for C header files. Issue found by checkpatch. Part of
Eudyptula Challenge.

Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>