zy1000: flush jtag buffer before changing speed

It is conceivable that there could be commands in the
queue when a speed change request comes in. Flush the
hw queue before changing speed. Not observed, found by
inspection.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

src/jtag/zy1000/zy1000.c

@@ -199,6 +199,9 @@ void zy1000_reset(int trst, int srst)
 
 int zy1000_speed(int speed)
 {
+	/* flush JTAG master FIFO before setting speed */
+	waitIdle();
+
 	if (speed == 0)
 	{
 		/*0 means RCLK*/