usb: xhci: remove unused member 'parent' in xhci_regset struct

The member @parent of xhci_regset struct is not used in fact,
so remove it

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>