mlxsw: reg: Allow to pass NULL pointer to mlxsw_reg_ralue_pack4/6()

In preparation for the change that is going to be done in the next
patch, allow to pass NULL pointer to mlxsw_reg_ralue_pack4() and
mlxsw_reg_ralue_pack6() helpers.

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