greybus: uart: wait for credits on shutdown

We should try to wait until all credits are accounted
for before returning from shutdown.

For this purpose add a helper function that will
wait on completion, and call it form the shutdown.
This helper will also be useful when "wait until sent" is
implemented.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>