xhci: Fix re-init on power loss after resume.

When a host controller has lost power during a suspend, we must
reinitialize it.  Now that the xHCI host has two roothubs, xhci_run() and
xhci_stop() expect to be called with both usb_hcd structures.  Be sure
that the re-initialization code in xhci_resume() mirrors the process the
USB PCI probe function uses.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>