usb: don't use bNbrPorts after initialization

After successful initialization hub->descriptor->bNbrPorts and
hub->hdev->maxchild are equal, but using hub->hdev->maxchild is
preferred because that value is explicitly used for initialization
of hub->ports[].

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>