libs/utils/results: fix Max slack computation
libs/utils/results: fix Max slack computation

The maximum slack is defined as:
  max_slack := configured_period - configured_run

This patch fixes its computation by using the proper columns of the RT-App
log file.

Signed-off-by: Juri Lelli <bdc6bcd4e8b34b4b92413d758024aeca6da0e459@arm.com>

arnoldlu/lisa,credp/lisa,mdigiorgio/lisa,ARM-software/lisa,bjackman/lisa,arnoldlu/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,JaviMerino/lisa,credp/lisa,joelagnel/lisa,mdigiorgio/lisa,ARM-software/lisa,joelagnel/lisa,bjackman/lisa,credp/lisa