In both 802.3z and SGMII modes we need to configure the MAC accordingly
to flip between Fiber and SGMII modes, and we need to read the MAC
status from the SGMII in-band control word.

Fixes: 0e01491d ("net: dsa: b53: Add SerDes support")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>