Reset the internal PCS state machine when changing interface mode.
This prevents confusing the state machine when changing interface
modes, e.g. from SGMII to 2500Base-X or vice-versa.
Fixes: 7e538372694b ("net: ethernet: mediatek: Re-add support SGMII") Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> Tested-by: Bjørn Mork <bjorn@mork.no> Signed-off-by: Daniel Golle <daniel@makrotopia.org> Signed-off-by: David S. Miller <davem@davemloft.net>