usb: xhci: Makefile: move xhci-pci and xhci-plat-hcd after xhci-hcd

Module xhci-pci and xhci-plat-hcd depend on xhci-hcd. Module xhci-hcd
should be put at a place before xhci-pci and xhci-plat-hcd. Otherwise,
xhci_hcd_init() might be executed after other functions in xhci-hcd if
they are all selected to be built in.

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