batman-adv: Remove too short %pM printk field width

The string representation for a mac address produced by %pM is 17
characters long. Left-aligning the output in a 15 character wide field
width %-15pM is therefore misleading and unnecessary.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>