staging: batman-adv: remove useless addr_to_string()

Since all *printf() methods in the kernel understand '%pM' modifier the
conversion to the string is useless beforehand.

Additionally this patch decreases batman_if structure by 20 bytes.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Marek Lindner <lindner_marek@yahoo.de>
Cc: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>