usb: xhci: remove dummy extra_priv_size for size of xhci_hcd struct

because hcd_priv_size is already size of xhci_hcd struct,
extra_priv_size is not needed anymore for MTK and tegra drivers.

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