realtek: pcs: rtl931x: drop USXGMII gating in setup_serdes

realtek: pcs: rtl931x: drop USXGMII gating in setup_serdes

The USXGMII_10GDXGMII and USXGMII_10GQXGMII early-return was added
when the submode register was not yet programmed, making those modes
effectively unconfigurable. With the submode now wired up at probe
time and written from the set_mode path, the gating is no longer
needed.

Keep the XSGMII gate - RTL8218D/E bring-up through the proprietary
10G SGMII path is still unimplemented - and rewrite the surrounding
comment accordingly.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23120
Signed-off-by: Robert Marko <robimarko@gmail.com>