usb: chipidea: support debugfs without CONFIG_USB_CHIPIDEA_DEBUG

Since we need to mount debugfs to show/store the things we
want to debug, it is duplicated to add another configuration
to enable it. Meanwhile, with CONFIG_USB_CHIPIDEA_DEBUG,
we can't support chipidea debugfs at runtime.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Cc: Jun Li <jun.li@freescale.com>
Signed-off-by: Peter Chen <peter.chen@kernel.org>