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

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

Fixes: dfba2174 ("usb: xhci: Add DbC support in xHCI driver")
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: NLu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: NPhilippe Ombredanne <pombredanne@nexB.com>
Signed-off-by: NMathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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
  *