batman-adv: use seq_puts instead of seq_printf when the format is constant

As reported by checkpatch, seq_puts has to be preferred with
respect to seq_printf when the format is a constant string
(no va_args)

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>