The controller driver may be NULL if the controller device
is the middle device between platform device and roothub.
This middle device may not need a device driver due to all
hardware control can be at platform device driver, this
platform device is usually a dual-role USB controller device.

The benefit of using this middle device is we can keep both
controller device's private data (known as struct usb_hcd)
for USB core use, and platform device's private data for
platform driver use.
Signed-off-by: NPeter Chen <peter.chen@nxp.com>
Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>