USB: serial: ftdi_sio: only allow valid latency timer values

Valid latency timer values are between 1 ms and 255 ms in 1 ms steps.
The store function for the "latency_timer" device attribute currently
allows any value, although only the lower 16 bits will be sent to the
device, and the device only stores the lower 8 bits.  The hardware
appears to accept the (invalid) value 0 and treats it the same as 1
(resulting in a latency of 1 ms).

Change the latency_timer_store() function to accept only the values 0 to
255, returning an error -EINVAL for out-of-range values.  Call
kstrtou8() to parse the integer instead of the obsolete
simple_strtoul().

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ftdi_sio.c

@@ -1716,9 +1716,12 @@ static ssize_t latency_timer_store(struct device *dev,
 {
 	struct usb_serial_port *port = to_usb_serial_port(dev);
 	struct ftdi_private *priv = usb_get_serial_port_data(port);
-	int v = simple_strtoul(valbuf, NULL, 10);
+	u8 v;
 	int rv;
 
+	if (kstrtou8(valbuf, 10, &v))
+		return -EINVAL;
+
 	priv->latency = v;
 	rv = write_latency_timer(port);
 	if (rv < 0)