net: dsa: mv88e6xxx: support in-band signalling on SGMII ports with external PHYs

If an external PHY is connected via SGMII and uses in-band signalling
then the auto-negotiated values aren't propagated to the port,
resulting in a broken link. See discussion in [0]. This patch adds
this propagation. We need to call mv88e6xxx_port_setup_mac(),
therefore export it from chip.c.

Successfully tested on a ZII DTU with 88E6390 switch and an
Aquantia AQCS109 PHY connected via SGMII to port 9.

[0] https://marc.info/?t=155130287200001&r=1&w=2



Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>