[correction] fix resume behavior for plugins with stack

This corrects my previous commit
https://github.com/xbmc/xbmc/pull/4616/files
I did paste in wrong place - "case 1: stacked ISOs" instead of "case 2: all other stacks"

Upd: Purpose of original commit: Take resume time for plugin items from the appropriate bookmark also for stack:// as it is done for not stacked items.