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: syzbot+f1842130bbcfb335bac1@syzkaller.appspotmail.com
Signed-off-by: Valentin Vidic <vvidic@valentin-vidic.from.hr>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/sr9800.c

@@ -335,7 +335,7 @@ static void sr_set_multicast(struct net_device *net)
 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);