realtek: pcs: rtl930x: move EEE to USXGMII configuration

realtek: pcs: rtl930x: move EEE to USXGMII configuration

The patch/config sequences for USXGMII modes contain a register write
which is explicitly labelled as "enable eee". To clean the sequences and
reduce duplication, move this specific write to the USXGMII
configuration function.

While at it, demystify this register write. From [1] we can see that
only a single bit needs to be set for controlling EEE instead of the
whole register. From testing it was seen that the register has a default
value of 0x445C after reset. Thus, there is no other operation hidden in
this write and we're safe to reduce it to setting just the EEE enable
bit.

[1] https://github.com/plappermaul/realtek-doc/blob/82af3a36b7f65dbe2158fef3a9b71e7aab94315e/sources/rtk-dms1250/include/hal/phy/rtl8295_reg_def.h

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22582
Signed-off-by: Robert Marko <robimarko@gmail.com>