Input: update SPDX tag for input-event-codes.h

Replace the
/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
with
/* SPDX-License-Identifier: GPL-2.0-only WITH Linux-syscall-note */

to help coreboot community consume this file without relaxing their
licensing checks.

Signed-off-by: Rajat Jain <rajatja@google.com>
Link: https://lore.kernel.org/r/20200329172513.133548-1-rajatja@google.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>