usbip: Correct maximum value of CONFIG_USBIP_VHCI_HC_PORTS

Now that usbip supports USB3, the maximum number of ports allowed
on a hub is 15 (USB_SS_MAXPORTS), not 31 (USB_MAXCHILDREN).

Reported-by: Gianluigi Tiesi <sherpya@netfarm.it>
Reported-by: Borissh1983 <borissh1983@gmail.com>
References: https://bugs.debian.org/878866
Fixes: 1c9de5bf

 ("usbip: vhci-hcd: Add USB3 SuperSpeed support")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>