usb: mtu3: power down port when power down device IP

When power down device IP, we can also power down device port,
then power on the port again when power on device ip, it's helpful
to make device ip enter ip sleep mode.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1623139069-8173-7-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>