usb: ehci: Fix test mode for connected ports

The EHCI controller has some very specific requirements for the USB 2.0
port test modes, which were not closely followed in the initial test
mode commit. It demands that the host controller is completely shut down
(all ports suspended, Run/Stop bit unset) when activating test mode, and
will not work on an already enumerated port.

This patch fixes that by introducing a new ehci_shutdown() function that
closely follows the procedure listed in EHCI 4.14. Also, when we have
such a function anyway, we might as well also use it in
usb_lowlevel_stop() to make the normal host controller shutdown cleaner.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>