[DECNET]: Fix refcount

From: Patrick Caulfield <patrick@tykepenguin.com>

This patch fixes a bug in the reference counting for the default
DECnet device.

If the device is changed, then the new device had its refcount
decremented rather than the old one!

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/decnet/dn_dev.c b/net/decnet/dn_dev.c
index d2ae989..a26ff9f 100644
--- a/net/decnet/dn_dev.c
+++ b/net/decnet/dn_dev.c
@@ -620,7 +620,7 @@
 	}
 	write_unlock(&dndev_lock);
 	if (old)
-		dev_put(dev);
+		dev_put(old);
 	return rv;
 }