Staging: batman-adv: Avoid rounding issues for local hna timeout

With the current default values, this patch is not critical, as
LOCAL_HNA_TIMEOUT is a multiple of 1000 anyway. However, if someone
would like to change this #define, the person could have some
unexpected rounding issues. Therefore doing the multiplication before
the division now.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>