net: fec: use mac-managed PHY PM

Use the new mac_managed_pm flag to work around an issue with KSZ8081 PHY
that becomes unstable when a soft reset is triggered during aneg.

Reported-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Tested-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>