lan78xx: Correctly indicate invalid OTP

BugLink: http://bugs.launchpad.net/bugs/1768429

[ Upstream commit 4bfc3380 ]

lan78xx_read_otp tries to return -EINVAL in the event of invalid OTP
content, but the value gets overwritten before it is returned and the
read goes ahead anyway. Make the read conditional as it should be
and preserve the error code.

Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/net/usb/lan78xx.c

@@ -618,7 +618,8 @@ static int lan78xx_read_otp(struct lan78xx_net *dev, u32 offset,
 			offset += 0x100;
 		else
 			ret = -EINVAL;
-		ret = lan78xx_read_raw_otp(dev, offset, length, data);
+		if (!ret)
+			ret = lan78xx_read_raw_otp(dev, offset, length, data);
 	}
 
 	return ret;