usb: xhci-pci: add support for hosts with zero USB3 ports

usb: xhci-pci: add support for hosts with zero USB3 ports

Add xhci support for PCI hosts that have zero USB3 ports.
Avoid creating a shared Host Controller Driver (HCD) when there is only
one root hub. Additionally, all references to 'xhci->shared_hcd' are now
checked before use.

Only xhci-pci.c requires modification to accommodate this change, as the
xhci core already supports configurations with zero USB3 ports. This
capability was introduced when xHCI Platform and MediaTek added support
for zero USB3 ports.

Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220181
Tested-by: Nick Nielsen <nick.kainielsen@free.fr>
Tested-by: grm1 <grm1@mailbox.org>
Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20250917210726.97100-4-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>