siimage: use ->pio_mode value to determine pair device speed

Use the current PIO mode value instead of the physical maximum one
of the pair device on the port to determine PIO commmand timings used
for both devices on the port.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>