usb/mtu3: power down device ip at setup

Originally, when dr_mode is USB_DR_MODE_HOST, it didn't power down device ip,
so host ip sleep will fail at ssusb_host_disable.

Power down device ip at ssusb_host_setup.

Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Hsin-Yi, Wang <hsinyi@chromium.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>