realtek: pcs: rtl931x: quit setup_serdes early on USXGMII mode

realtek: pcs: rtl931x: quit setup_serdes early on USXGMII mode

In rtpcs_931x_setup_serdes, quit early on USXGMII mode. This restores
the behaviour introduced in c18476d0c5 to prevent the current buggy
procedure to destroy a working configuration established by U-Boot
before.

Also include the valuable comment from the code to keep the information.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20369
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>