mlxsw: spectrum: Export RALUE pack helper and use it from IPIP

As the RALUE packing is going to be put into op, make the user from
IPIP code use the same helper as the router code does.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>