usb: xhci: dbc: Add SPDX identifiers to dbc files

Update the xhci dbc files with the correct SPDX license identifiers.

Fixes: dfba2174dc42 ("usb: xhci: Add DbC support in xHCI driver")
Cc: Philippe Ombredanne <[e-mail]>
Signed-off-by: Lu Baolu <[e-mail]>
Reviewed-by: Philippe Ombredanne <[e-mail]>
Signed-off-by: Mathias Nyman <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/host/xhci-dbgcap.c

···
+// SPDX-License-Identifier: GPL-2.0
 /**
  * xhci-dbgcap.c - xHCI debug capability support
  *

drivers/usb/host/xhci-dbgcap.h

···
-
+/* SPDX-License-Identifier: GPL-2.0 */
 /**
  * xhci-dbgcap.h - xHCI debug capability support
  *

drivers/usb/host/xhci-dbgtty.c

···
+// SPDX-License-Identifier: GPL-2.0
 /**
  * xhci-dbgtty.c - tty glue for xHCI debug capability
  *