The 'usb-redir' device requires the USB core code to work. Do not
link it when there is no USB support. This fixes:

  $ qemu-system-tricore -M tricore_testboard -device usb-redir
  qemu-system-tricore: -device usb-redir: No 'usb-bus' bus found for device 'usb-redir'

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191231183216.6781-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>