mlxsw: core_acl: Avoid defining static variable in header file

The static array 'mlxsw_afk_element_infos' in 'core_acl_flex_keys.h' is
copied to each file that includes the header, but not all use it. This
results in the following warnings when compiling with W=1:

drivers/net/ethernet/mellanox/mlxsw//core_acl_flex_keys.h:76:44:
warning: ‘mlxsw_afk_element_infos’ defined but not used
[-Wunused-const-variable=]

One way to suppress the warning is to mark the array with
'__maybe_unused', but another option is to remove it from the header
file entirely.

Change 'struct mlxsw_afk_element_inst' to store the key to the array
('element') instead of the array value keyed by 'element'. Adjust the
different users accordingly.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>