realtek: pcs: make use of SerDes struct in set_autoneg

realtek: pcs: make use of SerDes struct in set_autoneg

Also switch set_autoneg (and related helper rtpcs_sds_modify) to the
SerDes struct instead of the plain SerDes id by using just the reference
to the SerDes instance instead of (ctrl, sds_id) tuple. This completes
the transition.

Signed-off-by: Jonas Jelonek <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21146
Signed-off-by: Robert Marko <[email protected]>
(cherry picked from commit d8dd8bd88f264d39f8c8c1cff056b4ec32a985bb)