staging: rtl8192e: add SPDX GPL-2.0 license identifier

Add the SPDX GPL-2.0 license identifier to fix checkpatch.pl warning
Issue found by checkpatch.pl warning:
"WARNING: Missing or malformed SPDX-License-Identifier tag in line 1"
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8192e/dot11d.c

+// SPDX-License-Identifier: GPL-2.0
 /******************************************************************************
  * Copyright(c) 2008 - 2010 Realtek Corporation. All rights reserved.
  *