realtek: pcs: assign SerDes reference upon PCS creation

realtek: pcs: assign SerDes reference upon PCS creation

Upon creation of a phylink_pcs instance by calling rtpcs_create, assign
a reference to the corresponding SerDes to the link structure. In the
next step, this should be used everywhere instead of the plain SerDes
number.

Rename the field used to hold the SerDes number from 'sds' to 'sds_num'
and name the new field 'sds' to make clear what is what.

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>