realtek: pcs: drop PCS creation without SerDes ref

realtek: pcs: drop PCS creation without SerDes ref

Since the beginning, the PCS driver had the ability to call its
rtpcs_create without a reference to a valid PCS node. A comment in the
code mentions that this is done for RTL838X and its built-in octa-PHY
which is connected directly instead of via a SerDes. Further
explanations are not provided.

Drop this ability and make the rtpcs_create call in the dsa driver
conditional. As the built-in PHY of RTL838X isn't attached to a SerDes,
there is no obvious point of having the PCS driver in that chain. The
ports are marked as internal and have no pcs-handle, thus no phylink_pcs
instance should be created.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21146
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit a58e41e522bf5b273fa626e03546b92141731d9c)