batman-adv: Remove uses of return value of seq_printf

This function is soon going to return void so remove the
return value use.

Convert the return value to test seq_has_overflowed() instead.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: David S. Miller <davem@davemloft.net>