usb: dwc3: create debugfs directory under usb root

Now the USB gadget subsystem can use the USB debugfs root directory,
so move dwc3's directory from the root of the debugfs filesystem into
the root of usb

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1573541519-28488-4-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>