greybus: loopback: round closest the sixth decimal

The original round was removed becaused it was rounding
the integer whereas we had decimals.
Round the sixth decimal.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>