greybus: loopback: Fix averaging

Currently, we are adding 0.5 to the average to round the average.
But we are using the remainder to calculate the decimal, so we do not
need to round the average.
In addition, use a u64 type for the remainder to avoid overflow
that might happen when stats->sum value is too big,
usually for requests per seconds and the throughput.

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