IB/mlx5: Use div64_u64 for num_var_hw_entries calculation

On i386:

ERROR: "__udivdi3" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!
ERROR: "__divdi3" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!

Fixes: f164be8c ("IB/mlx5: Extend caps stage to handle VAR capabilities")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reported-by: Alexander Lobakin <alobakin@dlink.ru>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>