net: dsa: lantiq_gswip: support offset of MII registers

net: dsa: lantiq_gswip: support offset of MII registers

The MaxLinear GSW1xx family got a single (R)(G)MII port at index 5 but
the registers MII_PCDU and MII_CFG are those of port 0.
Allow applying an offset for the port index to access those registers.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://patch.msgid.link/88145164c1f948e4ae9b04706f408359cf54223c.1756520811.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>