usb: acpi: Prevent null pointer dereference in usb_acpi_add_usb4_devlink()

As demonstrated by the fix for update_port_device_state,
commit 12783c0b9e2c ("usb: core: Prevent null pointer dereference in update_port_device_state"),
usb_hub_to_struct_hub() can return NULL in certain scenarios,
such as during hub driver unbind or teardown race conditions,
even if the underlying usb_device structure exists.

Plus, all other places that call usb_hub_to_struct_hub() in the same file
do check for NULL return values.

If usb_hub_to_struct_hub() returns NULL, the subsequent access to
hub->ports[udev->portnum - 1] will cause a null pointer dereference.

Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
Fixes: f1bfb4a6fed6 ("usb: acpi: add device link between tunneled USB3 device and USB4 Host Interface")
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20250417195032.1811338-1-chenyuan0y@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>