mlxsw: spectrum: Use PMTDB register to obtain split info

Newly introduced PMTDB register is there to provide all needed info
about particular requested port split configuration. Use it instead of
figuring the info out manually in the driver.

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>