mlxsw: spectrum_router: Constify struct devlink_dpipe_table_ops

'struct devlink_dpipe_table_ops' are not modified in this driver.

Constifying these structures moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  15557	    712	      0	  16269	   3f8d	drivers/net/ethernet/mellanox/mlxsw/spectrum_dpipe.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  15789	    488	      0	  16277	   3f95	drivers/net/ethernet/mellanox/mlxsw/spectrum_dpipe.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>