net: usb: sr9800: fix uninitialized local variable

Make sure res does not contain random value if the call to
sr_read_cmd fails for some reason.

Reported-by: [e-mail]
Signed-off-by: Valentin Vidic <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/usb/sr9800.c

···
 static int sr_mdio_read(struct net_device *net, int phy_id, int loc)
 {
 	struct usbnet *dev = netdev_priv(net);
-	__le16 res;
+	__le16 res = 0;
 
 	mutex_lock(&dev->phy_mutex);
 	sr_set_sw_mii(dev);