libnvdimm, pmem: Fix memcpy_mcsafe() return code handling in nsio_rw_bytes()

Commit 60622d68 "x86/asm/memcpy_mcsafe: Return bytes remaining"
converted callers of memcpy_mcsafe() to expect a positive 'bytes
remaining' value rather than a negative error code. The nsio_rw_bytes()
conversion failed to return success. The failure is benign in that
nsio_rw_bytes() will end up writing back what it just read.

Fixes: 60622d68 ("x86/asm/memcpy_mcsafe: Return bytes remaining")
Cc: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>