Devices are created for each port of the XUSB pad controller. Each USB 2
and USB 3 port can potentially have an associated VBUS power supply that
needs to be removed when the device is removed.

Since port devices never bind to a driver, the driver core will not get
to perform the cleanup of device-managed resources that usually happens
on driver unbind.

Now, the driver core will also perform device-managed resource cleanup
for driver-less devices when they are released. However, when a device
link is created between the regulator and the port device, as part of
regulator_get(), the regulator takes a reference to the port device and
prevents it from being released unless regulator_put() is called, which
will never happen.

Avoid this by using the non-device-managed API and manually releasing
the regulator reference when the port is unregistered.
Signed-off-by: Thierry Reding <treding@nvidia.com>