xhci: Index with a port array instead of PORTSC addresses.

In the upcoming patches, the roothub emulation code will need to return
port status and port change buffers based on whether they are called with
the xHCI USB 2.0 or USB 3.0 roothub.  To facilitate that, make the roothub
code index into an array of port addresses with wIndex, rather than
calculating the address using the offset and the address of the PORTSC
registers.  Later we can set the port array to be the array of USB 3.0
port addresses, or the USB 2.0 port addresses, depending on the roothub
passed in.

Create a temporary (statically sized) port array and fill it in with both
USB 3.0 and USB 2.0 port addresses.  This is inefficient to do for every
roothub call, but this is needed for git bisect compatibility.  The
temporary port array will be deleted in a subsequent patch.

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