Correct target calculation in gen_stratum_work.

cgminer.c

@@ -4552,7 +4552,7 @@ static void gen_stratum_work(struct pool *pool, struct work *work)
 		quit(1,  "Failed to convert hash1 in gen_stratum_work");
 
 	/* Generate target as hex where 0x00000000FFFFFFFF is diff 1 */
-	diff64 = 0x00000000FFFFFFFFULL * diff;
+	diff64 = (1Ull << (31 + diff)) - 1;
 	diff64 = ~htobe64(diff64);
 	sprintf(target, "ffffffffffffffffffffffffffffffffffffffffffffffff");
 	buf = bin2hex((const unsigned char *)&diff64, 8);