net: phy: at803x: merge qca8081 slave seed function

merge the seed enablement and seed value configuration into
one function, since the random seed value is needed to be
configured when the seed is enabled.

Signed-off-by: Luo Jie <quic_luoj@quicinc.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>