net: dsa: sja1105: allow SGMII PCS configuration to be per port

The SJA1105 R and S switches have 1 SGMII port (port 4). Because there
is only one such port, there is no "port" parameter in the configuration
code for the SGMII PCS.

However, the SJA1110 can have up to 4 SGMII ports, each with its own
SGMII register map. So we need to generalize the logic.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>