xhci: Fix NULL pointer dereference when reading portli debugfs files

In the Linux kernel, the following vulnerability has been resolved:

xhci: Fix NULL pointer dereference when reading portli debugfs files

Michal reported and debgged a NULL pointer dereference bug in the

recently added portli debugfs files

Oops is caused when there are more port registers counted in

xhci->max_ports than ports reported by Supported Protocol capabilities.

This is possible if max_ports is more than maximum port number, or

if there are gaps between ports of different speeds the 'Supported

Protocol' capabilities.

In such cases port->rhub will be NULL so we can't reach xhci behind it.

Add an explicit NULL check for this case, and print portli in hex

without dereferencing port->rhub.