realtek: pcs: make use of SerDes struct in SerDes setup

Make use of the previously added SerDes struct in SerDes setup and all
functions in its call path by removing (ctrl, sds_num) being passed to
every function call and instead just pass the reference to the
corresponding SerDes instance.

Various SerDes calculations for even, odd and neighbor are unified by
switching to previously introduced helpers.

Signed-off-by: Jonas Jelonek <[email protected]>
Link: openwrt/openwrt#21146
Signed-off-by: Robert Marko <[email protected]>
(cherry picked from commit 1fccb0e)

Author: jonasjelonek