usb: usb251xb: Add USB251x specific port count setting

USB251xb as well as USB2517 datasheet states, that all these
hubs differ by number of ports declared as the last digit in the
model name. So USB2512 got two ports, USB2513 - three, and so on.
Such setting must be reflected in the device specific data
structure and corresponding dts property should be checked whether
it doesn't get out of available ports.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>