xhci: Fix oops caused by more USB2 ports than USB3 ports.

The code to set the device removable bits in the USB 2.0 roothub
descriptor was accidentally looking at the USB 3.0 port registers
instead of the USB 2.0 registers.  This can cause an oops if there are
more USB 2.0 registers than USB 3.0 registers.

This should be backported to kernels as old as 2.6.39, that contain the
commit 4bbb0ace "xhci: Return a USB 3.0
hub descriptor for USB3 roothub."

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org