net: dsa: microchip: Disable RGMII in-band status on KSZ9893

We can't assume that the link partner supports the in-band status
reporting which is enabled by default on the KSZ9893 when using RGMII
for the upstream port.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>