This wrapper for machine_usb(current_machine) is not necessary,
replace all usages of usb_enabled() with machine_usb().

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: qemu-arm@nongnu.org
Cc: qemu-ppc@nongnu.org
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-id: 1465419025-21519-3-git-send-email-ehabkost@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>