realtek: dsa: remove sds_num entirely

realtek: dsa: remove sds_num entirely

After having moved RTL93XX SerDes configuration from PHY to PCS driver,
the DSA driver doesn't need to know about SerDes explicitly anymore.

Although RTL83XX SerDes is still partly managed within the DSA driver,
it doesn't make use of the sds_num property/field. RTL93XX was the only
user of this right now.

Thus, we can just remove the remaining 'sds_num' code which doesn't
serve any purpose anymore.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20577
Signed-off-by: Robert Marko <robimarko@gmail.com>