sfc: add mport lookup based on driver's mport data

Obtaining mport id is based on asking the firmware about it. This is
still needed for mport initialization itself, but once the mport data is
now kept by the driver, further mport id request can be satisfied
internally without firmware interaction.

Previous function is just modified in name making clear the firmware
interaction. The new function uses the old name and looks for the data
in the mport data structure.

Signed-off-by: Alejandro Lucero <alejandro.lucero-palau@amd.com>
Acked-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>