mlxsw: spectrum_router: Push out RALUE pack into separate helper

As the RALUE packing is going to be pushed into an op, in preparation
for that push the code into a separate function in the meantime.

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