batman-adv: Replace gw_reselect divisor with simple shift

The gw_factor is divided by BATADV_TQ_LOCAL_WINDOW_SIZE ** 2 * 64. But the
rest of the calculation has nothing to do with the tq window size and
therefore the calculation is just (tmp_gw_factor / (64 ** 3)).

Replace it with a simple shift to avoid a costly 64-bit divide when the
max_gw_factor is changed from u32 to u64. This type change is necessary
to avoid an overflow bug.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>