realtek: pcs: rtl931x: merge helper into mode setting

realtek: pcs: rtl931x: merge helper into mode setting

Merge the 'rtpcs_931x_sds_disable' helper into the MII mode setting and
use that instead. The helper was essentially doing same just with a
value for OFF state.

Moreover, the name of the helper was confusing. It implied it disables
the whole SerDes. However, this is used in Fiber mode setting and thus,
cannot completely disable the SerDes.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20736
Signed-off-by: Robert Marko <robimarko@gmail.com>