generic: disable SGMII in-band AN for RealTek 2.5G PHYs

generic: disable SGMII in-band AN for RealTek 2.5G PHYs

MAC drivers don't use SGMII in-band autonegotiation unless told to do so
in device tree using 'managed = "in-band-status"'. When using MDIO to
access a PHY, in-band-status is unneeded as we have link-status via
MDIO. Switch off SGMII in-band autonegotiation using magic values.

Reported-by: Chen Minqiang <ptpt52@gmail.com>
Reported-by: Chukun Pan <amadeus@jmu.edu.cn>
Reported-by: Yevhen Kolomeiko <jarvis2709@gmail.com>
Tested-by: Yevhen Kolomeiko <jarvis2709@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>