usb/ohci: Fix crash with when specifying too many num-ports

QEMU currently crashes when an OHCI controller is instantiated with
too many ports, e.g. "-device pci-ohci,num-ports=100,masterbus=1".
Thus add a proper check in usb_ohci_init() to make sure that we
do not use more than OHCI_MAX_PORTS = 15 ports here.

Ticket: https://bugs.launchpad.net/qemu/+bug/1581308
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1463995387-11710-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
