net: stmmac: dwc-qos: calibrate tegra with mdio bus idle

Thierry states that there are prerequists for Tegra's calibration
that should be met before starting calibration - both the RGMII and
MDIO interfaces should be idle.

This commit adds the necessary MII bus locking to ensure that the MDIO
interface is idle during calibration.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patch.msgid.link/E1u7EYR-001ZAS-Cr@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>