realtek: pcs: drop unneeded SerDes number range checks

realtek: pcs: drop unneeded SerDes number range checks

By using references to pre-initiated SerDes instances instead of plain
SerDes number, there is no need to check for the range anymore in
various places. During driver/pcs init it is ensured that only valid
SerDes will reach the configuration functions.

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 18eea0548356454376ad3c60145446060325ace3)