USB: serial: f81534: detect errors from f81534_logic_to_phy_port()

With gcc 4.1.2:

    drivers/usb/serial/f81534.c: In function ‘f81534_port_probe’:
    drivers/usb/serial/f81534.c:1250: warning: comparison is always false due to limited range of data type

f81534_logic_to_phy_port() may return a negative error value, which is
ignored by assigning it to u8 f81534_port_private.phy_num.

Use an intermediate variable of type int to fix this.
While at it, forward the actual error code instead of converting it to
-ENODEV, and drop the useless check for F81534_NUM_PORT, as the callee
always returns a valid port number in case of success.

Fixes: 0c9bd6004d258d46 ("USB: serial: add Fintek F81532/534 driver")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/f81534.c

@@ -1237,6 +1237,7 @@ static int f81534_attach(struct usb_serial *serial)
 static int f81534_port_probe(struct usb_serial_port *port)
 {
 	struct f81534_port_private *port_priv;
+	int ret;
 
 	port_priv = devm_kzalloc(&port->dev, sizeof(*port_priv), GFP_KERNEL);
 	if (!port_priv)
@@ -1246,10 +1247,11 @@ static int f81534_port_probe(struct usb_serial_port *port)
 	mutex_init(&port_priv->mcr_mutex);
 
 	/* Assign logic-to-phy mapping */
-	port_priv->phy_num = f81534_logic_to_phy_port(port->serial, port);
-	if (port_priv->phy_num < 0 || port_priv->phy_num >= F81534_NUM_PORT)
-		return -ENODEV;
+	ret = f81534_logic_to_phy_port(port->serial, port);
+	if (ret < 0)
+		return ret;
 
+	port_priv->phy_num = ret;
 	usb_set_serial_port_data(port, port_priv);
 	dev_dbg(&port->dev, "%s: port_number: %d, phy_num: %d\n", __func__,
 			port->port_number, port_priv->phy_num);