net: usb: sr9700: fix uninitialized variable use in sr_mdio_read

It could lead to error happen because the variable res is not updated if
the call to sr_share_read_word returns an error. In this particular case
error code was returned and res stayed uninitialized. Same issue also
applies to sr_read_reg.

This can be avoided by checking the return value of sr_share_read_word
and sr_read_reg, and propagating the error if the read operation failed.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: c9b37458 ("USB2NET : SR9700 : One chip USB 1.1 USB2NET SR9700Device Driver Support")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/sr9700.c

@@ -179,6 +179,7 @@ static int sr_mdio_read(struct net_device *netdev, int phy_id, int loc)
 	struct usbnet *dev = netdev_priv(netdev);
 	__le16 res;
 	int rc = 0;
+	int err;
 
 	if (phy_id) {
 		netdev_dbg(netdev, "Only internal phy supported\n");
@@ -189,11 +190,17 @@ static int sr_mdio_read(struct net_device *netdev, int phy_id, int loc)
 	if (loc == MII_BMSR) {
 		u8 value;
 
-		sr_read_reg(dev, SR_NSR, &value);
+		err = sr_read_reg(dev, SR_NSR, &value);
+		if (err < 0)
+			return err;
+
 		if (value & NSR_LINKST)
 			rc = 1;
 	}
-	sr_share_read_word(dev, 1, loc, &res);
+	err = sr_share_read_word(dev, 1, loc, &res);
+	if (err < 0)
+		return err;
+
 	if (rc == 1)
 		res = le16_to_cpu(res) | BMSR_LSTATUS;
 	else