batman-adv: unify and properly print hex values

Values are printed in hexadecimal format in several points in the
code, but they are not printed using the same format string.

This patches unifies the format used for such numbers so that they
look the same everywhere.

Given the fact that all the variables printed as hexadecimal are 16
bit long, this is the chosen printing format: %#.4x

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