realtek: pcs: use per-variant SerDes count

realtek: pcs: use per-variant SerDes count

Use a separate configuration field for the number of SerDes for each
variant of the Realtek Otto family. Add this field to the config
structure, assign it and use it during driver probe. This narrows
possible error cases and is needed for upcoming extensions.

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 6cf33aacfe9d01903f1662795a28c5a7fc6e2fb7)