sysbus-ohci: Remove user_creatable flag

sysbus-ohci needs to be mapped and wired by device or board code,
and won't work with -device. Remove the user_creatable flag from
the device class.

Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-19-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>