This patch cleanups the hcd private size to suitable size.
The previous code has "sizeof(struct xhci_hcd *)" in xhci_hc_driver
as hcd_priv_size and sizeof(struct xhci_hcd) in xhci_plat_overrides
or xhci_pci_overrides as extra_priv_size. However, the xhci driver
uses a "sizeof(struct xhcd_hcd)" memory space in each hcd
(main_hcd and shared_hcd) actually.