batman-adv: don't switch byte order too often if not needed

If possible, operations like ntohs/ntohl should not be
performed too often. Use a variable to locally store the
converted value and then use it.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>