USB: host: xhci-debugfs: add SPDX lines

The license text was removed from these files, yet they somehow missed
the big "add SPDX tags to all files" sweep due to them being new.  So
add the proper SPDX license tag to them, based on the original license
text in the file.
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-debugfs.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * xhci-debugfs.c - xHCI debugfs interface
  *

drivers/usb/host/xhci-debugfs.h

+// SPDX-License-Identifier: GPL-2.0
 /*
  * xhci-debugfs.h - xHCI debugfs interface
  *