greybus: loopback: use separate attribute macro for average

Define a separate macro for displaying the average of the samples
collected.  This will be used so we can calculate the average only
when requested, rather than every time a new value gets recorded.

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>