greybus: loopback: use u32 for stats update

The only values supplied to gb_loopback_update_stats() are 32-bits,
so change the type of the second argument to reflect that.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>