Fix completely broken ESP8266 miner (ver. 3.33) - #1586

Can you provide more details?

b54d33b this patch introduced MiningJob struct, and job parsing functionality was mainly moved there. But unsigned int difficulty = 0; global var is still here (not updated anymore), so for (unsigned int duco_numeric_result = 0; duco_numeric_result < difficulty; duco_numeric_result++) { never enters the loop body (as difficulty is now always zero). So, fixing that by using correct job.difficulty. But we can't remove difficulty var because it's used by web dashboard - so I still save last job parsing result to this var.