succd: Fix -KEC1 relay board updates

The accesses to -KEC1 always time out on the attempt to update the
output values.  We noticed that this is related to the timing between
the reading of the inputs and the following write to the outputs.

Fix -KEC1 accesses by waiting before sending the next request to the
board after receiving the reply for the previous one.

succbone/succd/modbus.go

@@ -101,6 +101,11 @@ func (d *daemon) modbusUpdate() {
 		// TODO: Input mapping goes here
 	}
 
+	// We must wait between reading and writing to the -KEC1 relay board
+	// because otherwise it chokes and times out the write registers
+	// command.
+	time.Sleep(time.Millisecond * 10)
+
 	// KFA1-KFA8
 	var relayState [8]bool
 	d.mu.Lock()