tools/testing/nvdimm: fix return code for unimplemented commands

The implementation for the new "DIMM Flags" DSM relies on the -ENOTTY
return code to indicate that the flags are unimplimented and to fall
back to a safe default.  As is the -ENXIO error code erroneoously
indicates to fail enabling a BLK region.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>