CVE-2026-43431: 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->maxports than ports reported by Supported Protocol capabilities. This is possible if maxports 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.