realtek: pcs: avoid unneeded SerDes reconfiguration

realtek: pcs: avoid unneeded SerDes reconfiguration

Avoid to reconfigure a SerDes when it has been configured for a mode
before. This usually applies to switches which drive multiple ports on a
single SerDes. For those, the phylink subsystem triggers PCS
configuration everytime although it's a single SerDes. For example, on
switches with XSGMII-connected RTL8218D the particular SerDes is
configured eight times but only a single run is needed.

Add a proper check to pcs_config which checks the mode stored in the
SerDes instance against the requested mode. Other 'settings' should be
executed though, e.g. setting autoneg. While at it, drop the check if
there is a 'setup_serdes' implementation. It's just a driver-internal
interface and all variants implement this now.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21763
Signed-off-by: Robert Marko <robimarko@gmail.com>