greybus: loopback: record 32-bit min and max

The minimum and maximum values for stats values are always 32 bits.
Change the type for these fields to reflect this.

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>