[ipv6] Fix uninitialised-variable warning

Fix uninitialised-variable warning reported by gcc 4.5.2.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ndp.c

 		remaining -= option_len;
 	}
 
+	/* Success */
+	rc = 0;
+
  done:
 	free_iob ( iobuf );
 	return rc;