net: stmmac: dwc-qos-eth: move MDIO bus locking into stmmac_mdio

net: stmmac: dwc-qos-eth: move MDIO bus locking into stmmac_mdio

Rather than dwc-qos-eth manipulating the MDIO bus lock directly, add
helpers to the stmmac MDIO layer and use them in dwc-qos-eth. This
improves my commit 87f43e6f06a2 ("net: stmmac: dwc-qos: calibrate tegra
with mdio bus idle").

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1v945J-0000000AmeJ-1GOb@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>