realtek: do not reset SerDes on link change

realtek: do not reset SerDes on link change

Do not reset the RTL930x SerDes on link changes, instead set up
the SDS with internal PHYs for the SFP+ ports only.
This fixes the 8 1GBit ports on the Zyxel XGS1250 which
do not work without this patch.

A complete SerDes reset was performed on all SerDes links. For copper
1Gbit ports, this is commonly a single XGMII link to an RTL8218D. There
is however no support for setting up the XGMII link on RTL9300/RTL9310,
thereby wiping the (RX/TX) setup done by u-boot and breaking the 1GBit
ports. No SerDes reset should be done for these links.

The handling of SGMII/HiSGMII, 1000BX or 10GR links is actually entirely
different. All these modes need to be suitably RX calibrated and the
pre- main and post- amplifiers set up properly for TX.

The 10GBit SFP+ fiber links are recalibrated instead of reset, which
e.g. is necessary when someone pulls a module out and puts another in.
This makes swapping out 10GBit fiber modules possible. 1GBit modules are
not yet supported, nor any modules with an internal phy.

Tested-by: Stijn Segers <foss@volatilesystems.org>
Signed-off-by: Birger Koblitz <git@birger-koblitz.de>
[rewrite commit message based on discussion]
Link: http://lists.infradead.org/pipermail/openwrt-devel/2022-May/038623.html
Signed-off-by: Sander Vanheule <sander@svanheule.net>