Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32f1/STM32F101/ETHERNET_MAC/MACMIIDR#0x0
Ethernet MAC MII data register (ETH_MACMIIDR)
MII data
https://github.com/modm-io/cmsis-svd-stm32