Staging: batman-adv: kfree_skb() in interface_tx() in error case

As we always return that the we consumed the skb, we should also free the skb
in the case of an error.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>