trivial: add __init macro/ fix of __exit macro location in ipmi_poweroff.c

Trivial patch which adds the __init to the module_init function of
drivers/char/ipmi/ipmy_poweroff.c and corrects the location of __exit for the
cleanup function.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Acked-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>