batman-adv: tp_meter: mark init function with __init

batadv_tp_meter_init() is invoked in batadv_init() only
which is marked with __init.

For this reason batadv_tp_meter_init() can be marked with
__init as well and dropped after module load.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>