realtek: pcs: introduce per-variant SerDes deactivate helpers

realtek: pcs: introduce per-variant SerDes deactivate helpers

Add rtpcs_{838x,930x,931x}_sds_deactivate() helpers that each encapsulate
the variant-specific "make SerDes inert before reconfigure" sequence, and
replace the inline calls at the start of each setup_serdes with a single
call to the new helper:

 - 838x: wraps rtpcs_838x_sds_power(sds, false)
 - 930x: wraps rtpcs_930x_sds_set_mode(sds, RTPCS_SDS_MODE_OFF)
 - 931x: rtpcs_931x_sds_power(sds, false) + rtpcs_931x_sds_set_mode(sds,
         RTPCS_SDS_MODE_OFF)

RTL839x has no deactivate step to factor out and is left unchanged.

This is a pure refactor: same register writes, same order, same return-
value handling at the call site. The helpers give each variant a named
hook for the deactivate phase and prepare for a subsequent commit that
promotes it to an rtpcs_sds_ops entry and hoists the call site into
rtpcs_pcs_config.

Link: https://github.com/openwrt/openwrt/pull/23513
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>