mlxsw: spectrum: Use PLLP to get front panel number and split number

Instead of relying on the values coming from the PMLP register, use PLLP
to get the information about port front panel number and split number.

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