mlxsw: spectrum_router: Pass destination IP as a pointer to mlxsw_reg_ralue_pack4()

Instead of passing destination IP as a u32 value, pass it as pointer to
u32. Avoid using local variable for the pointer store.

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