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: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Philippe Ombredanne <pombredanne@nexB.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>