xhci: Make roothub functions deal with device removal.

Return early in the roothub control and status functions if the xHCI host
controller is not electrically present in the system (register reads
return all "fs").  This issue only shows up when the xHCI driver registers
two roothubs and the host controller is removed from the system.

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