wifi_statistics: remove auto-generated mode.c

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
wifi_statistics: add compat code for pre-3.9 kernel

Kernel prior to 3.9.0 used an extra argument for various
hlist_for_each_entry* functions which was removed. Add some
compat code for older kernel and update source for new functions.
Also outsource compat code into own header file compat.h.

Compat code for hlist functions as well as the compat.h idea comes
from the batman-adv project.

Reported-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>