greybus: uart: Implement flush_buffer

Data may be held pening in the hardware because of flow
control mechanisms. When the port is closed, we need to flush
all data that was not sent.

For this, use the greybus message GB_UART_TYPE_FLUSH_FIFOS
which will flush all data queued on the module but not
yet sent on the data line.

Suggested-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>