batman-adv: Add hard_iface specific sysfs wrapper macros for UINT

This allows us to easily add a sysfs parameter for an
unsigned int later, which is not for a batman mesh interface
(e.g. bat0), but for a common interface instead. It allows
reading and writing an atomic_t in hard_iface (instead of
bat_priv compared to the mesh variant).

Developed by Linus during a 6 months trainee study period in
Ascom (Switzerland) AG.

Signed-off-by: Linus Luessing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
[antonio@open-mesh.com: rename functions and move macros]
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>