usb: mtu3: support suspend/resume for device mode

Support suspend/resume for device mode if the device is not
connected with a host, otherwise reject to suspend.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1626340078-29111-13-git-send-email-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>