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.

Reported-by: Michal Pecio <michal.pecio@gmail.com>
Closes: https://lore.kernel.org/linux-usb/20260304103856.48b785fd.michal.pecio@gmail.com
Fixes: 384c57ec7205 ("usb: xhci: Add debugfs support for xHCI Port Link Info (PORTLI) register.")
Cc: stable@vger.kernel.org
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://patch.msgid.link/20260304223639.3882398-4-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>