usb: core: hub: controller driver name may be NULL

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: Peter Chen <peter.chen@nxp.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>