batman-adv: replace non-atomic hardif config fields with (READ|WRITE)_ONCE

batman-adv: replace non-atomic hardif config fields with (READ|WRITE)_ONCE

The hardif configuration values are only accessed as plain loads/stores and
do not require full atomic_t semantics. Convert these fields to native
integer types and replace their users with READ_ONCE()/WRITE_ONCE() to
avoid load/store tearing.

Signed-off-by: Sven Eckelmann <sven@narfation.org>