ethtool: fix a memory leak in ethnl_default_start()

If ethnl_default_parse() fails then we need to free a couple
memory allocations before returning.

Fixes: 728480f1 ("ethtool: default handlers for GET requests")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>