On STM32, in case of PHY failure, usually because of the lack of 25/50 MHz...

On STM32, in case of PHY failure, usually because of the lack of 25/50 MHz REFCLK input, the Ethernet block would lock up in a while loop waiting for readiness, which will never happened.  This prevented the board from starting at all in the case of a PHY failure.  This commit adds a retry loop to avoid bricking the board if the Ethernet block is unable to start.