esp: implement FIFO flush command

At this point it is now possible to properly implement the FIFO flush command
without causing guest errors.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-32-mark.cave-ayland@ilande.co.uk>