usb: ehci: only shutdown opened controller

If the usb controller is not running, no need to shutdown it,
otherwise `usb stop` complains about:
"EHCI failed to shut down host controller".

To i.MX7D SDB, there are two usb ports, one Host, one OTG.
If we only plug one udisk to the Host port and then `usb start`,
the OTG controller for OTG port does not run actually. Then,
if `usb stop`, the OTG controller for OTG port will also be
shutdown, but it is not running.

This patch adds a check to only shutdown the running controller.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: "Stefan Brüns" <stefan.bruens@rwth-aachen.de>
Cc: Stephen Warren <swarren@nvidia.com>