net: phy: at803x: enable qca8081 slave seed conditionally

qca8081 is the single port PHY, the slave prefer mode is used
by default.

if the phy master perfer mode is configured, the slave seed
configuration should not be enabled, since the slave seed
enablement is for making PHY linked as slave mode easily.

disable slave seed if the master mode is preferred.

Signed-off-by: Luo Jie <quic_luoj@quicinc.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>