USB: enclose USB_XHCI_HCD related symbols within a if USB_XHCI_HCD block

This patch encloses all symbols depending on USB_XHCI_HCD within an if
USB_XHCI_HCD / endif block.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>