usb: chipidea: debug: create debugfs directory under usb root

Move it'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/1573541519-28488-2-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>