mlx4_en: fix a build error on 32bit arches

commit b6c39bfc ("net/mlx4_en: Add a service task")
added a build error on 32bit arches.

ERROR: "__udivdi3" [drivers/net/ethernet/mellanox/mlx4/mlx4_en.ko]
undefined!

Fix this problem by using do_div()

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>