xhci: Store the "real" root port number.

Since the xHCI driver now has split USB2/USB3 roothubs, devices under each
roothub can have duplicate "fake" port numbers.  For the next set of
patches, we need to keep track of the "real" port number that the xHCI
host uses to index into the port status arrays.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>