usbnet/smsc75xx: silence uninitialized variable warning

If the fn() calls fail then "buf" is uninitialized.  Just return early
in that situation.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/smsc75xx.c

@@ -99,9 +99,11 @@ static int __must_check __smsc75xx_read_reg(struct usbnet *dev, u32 index,
 	ret = fn(dev, USB_VENDOR_REQUEST_READ_REGISTER, USB_DIR_IN
 		 | USB_TYPE_VENDOR | USB_RECIP_DEVICE,
 		 0, index, &buf, 4);
-	if (unlikely(ret < 0))
+	if (unlikely(ret < 0)) {
 		netdev_warn(dev->net, "Failed to read reg index 0x%08x: %d\n",
 			    index, ret);
+		return ret;
+	}
 
 	le32_to_cpus(&buf);
 	*data = buf;