greybus: loopback: use U64_MAX for initialization

Use the largest representable value when initializing the "min"
field when resetting loopback statistics.

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>