staging: comedi: comedi_usb.h: SPDX License Identifier is added in the first line

SPDX License Identifier is added in form of a comment.

Signed-off-by: Bhanusree Pola <bhanusreemahesh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>