Use DeviceClass rather than SysBusDeviceClass in
milkymist_softusb_class_init().

Cc: michael@walle.cc
Signed-off-by: NMao Zhongyi <maozhongyi@cmss.chinamobile.com>
Signed-off-by: NZhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181130093852.20739-9-maozhongyi@cmss.chinamobile.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>