drivers: eth: fix stm32_hal PHY address resolution

When the PHY is hardware designed to communicate on address '1', the mdio
node of the device tree is labeled 'ethernet-phy@1'.
Therefore, STM32 HAL must adapt to resolve the appropriate DT node:
'ethernet_phy_1'.

Signed-off-by: Alex Fabre <alex.fabre@rtone.fr>