USB: serial: ftdi_sio: don't access latency timer on old chips

The latency timer was introduced with the FT232BM and FT245BM chips.  Do
not bother attempting to read or write it for older chip versions.
Signed-off-by: NIan Abbott <abbotti@mev.co.uk>
Signed-off-by: NJohan Hovold <johan@kernel.org>

drivers/usb/serial/ftdi_sio.c

@@ -1406,6 +1406,9 @@ static int write_latency_timer(struct usb_serial_port *port)
 	int rv;
 	int l = priv->latency;
 
+	if (priv->chip_type == SIO || priv->chip_type == FT8U232AM)
+		return -EINVAL;
+
 	if (priv->flags & ASYNC_LOW_LATENCY)
 		l = 1;
 
@@ -1429,6 +1432,9 @@ static int read_latency_timer(struct usb_serial_port *port)
 	unsigned char *buf;
 	int rv;
 
+	if (priv->chip_type == SIO || priv->chip_type == FT8U232AM)
+		return -EINVAL;
+
 	buf = kmalloc(1, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;