mlxsw: spectrum_acl_flex_keys: Constify struct mlxsw_afk_element_inst

'struct mlxsw_afk_element_inst' are not modified in these drivers.

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

Update a few functions and struct mlxsw_afk_block accordingly.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   4278	   4032	      0	   8310	   2076	drivers/net/ethernet/mellanox/mlxsw/spectrum_acl_flex_keys.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   7934	    352	      0	   8286	   205e	drivers/net/ethernet/mellanox/mlxsw/spectrum_acl_flex_keys.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/8ccfc7bfb2365dcee5b03c81ebe061a927d6da2e.1727541677.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jakub Kicinski <kuba@kernel.org>