core: Display difficulty in cgminer style, not true network difficulty (for consistency within the interface).

This regression was introduced by commit 3f6b9d6796.

cgminer.c

@@ -3758,7 +3758,7 @@ static void set_blockdiff(const struct work *work)
 	uint8_t pow = work->data[72];
 	int powdiff = (8 * (0x1d - 3)) - (8 * (pow - 3));
 	uint32_t diff32 = be32toh(*((uint32_t *)(work->data + 72))) & 0x00FFFFFF;
-	double numerator = 0xFFFFULL << powdiff;
+	double numerator = 0xFFFFFFFFULL << powdiff;
 	double ddiff = numerator / (double)diff32;
 
 	if (unlikely(current_diff != ddiff)) {