mellanox: mlxsw: Use '%zx' to print size_t format

Use '%zx' to print size_t format in order to fix the following build warning:

drivers/net/ethernet/mellanox/mlxsw/item.h:65:3: warning: format '%lx' expects argument of type 'long unsigned int', but argument 6 has type 'size_t' [-Wformat=]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>