staging: greybus: fix SPDX License Identifier

Replace the SPDX identifier with standard C comment.
Reported by checkpatch.
Signed-off-by: Ioannis Valasakis <code@wizofe.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/control.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Greybus CPort control protocol
  *