tools/testing/nvdimm: Cleanup dimm index passing

The ND_CMD_CALL path only applies to the nfit_test0 emulated DIMMs.
Cleanup occurrences of (i - t->dcr_idx) since that offset fixup only
applies to cases where nfit_test1 needs a bus-local index.

Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>