realtek: pcs: move USXGMII submodes out of sds_mode enum

realtek: pcs: move USXGMII submodes out of sds_mode enum

Carrying all six USXGMII submodes as distinct rtpcs_sds_mode values
forced every switch/case over hw_mode to enumerate them individually,
even though almost none of that code actually cares which submode is
active; a couple of places on 930X do, but only to tell SXGMII/QXGMII
apart from the rest.

Collapse them into a single RTPCS_SDS_MODE_USXGMII and introduce
rtpcs_sds_usxgmii_submode for the cases that still need the
distinction, threading it explicitly through hw-mode selection and
set_hw_mode instead of overloading the mode enum for it.

Link: https://github.com/openwrt/openwrt/pull/24494
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>