mlxsw: core_acl: Use an array instead of a struct with a zero-length array

Suppresses the following coccinelle warning:

drivers/net/ethernet/mellanox/mlxsw/core_acl_flex_keys.c:139:3-7:
WARNING use flexible-array member instead

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