elf: Fix a DTV setup issue [BZ #27136]

The max modid is a valid index in the dtv, it should not be skipped.

The bug is observable if the last module has modid == 64 and its
generation is same or less than the max generation of the previous
modules.  Then dtv[0].counter implies dtv[64] is initialized but
it isn't. Fixes bug 27136.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>