Platform/RPi3: Gracefully disable USB channel after a timeout

When a timeout occurs, attempt to gracefully disable the channel.
Report a final status of EFI_TIMEOUT if the channel was disabled
successfully, or EFI_DEVICE_ERROR if we were unable to disable the
channel.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>