stm32/eth: Add warning for autonegotiation timeout fallback.

Print warning message when autonegotiation times out and code falls
back to 10Mbps Half-Duplex mode.

Issue: After 5-second autonegotiation timeout (PHY_AUTONEG_TIMEOUT_MS),
code silently falls back to 10Mbps Half-Duplex - 10x slower than typical
100Mbps Full-Duplex. Users experience slow network with no indication of
root cause.

Solution: Print warning message to console when fallback occurs:
"ETH: Autonegotiation timeout, using 10Mbps Half-Duplex"

Impact: Users can diagnose slow network performance and investigate
autonegotiation failures (bad cable, switch issues, etc.).

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl

ports/stm32/eth.c

@@ -837,6 +837,7 @@ void eth_phy_link_status_poll() {
             // If autoneg timed out and speed couldn't be read, use safe default (10Mbps Half)
             if (autoneg_timeout && phy_speed == 0) {
                 phy_speed = PHY_SPEED_10HALF;
+                mp_printf(&mp_plat_print, "ETH: Autonegotiation timeout, using 10Mbps Half-Duplex\n");
             }
 
             // Set flag to prevent IRQ handler from processing packets during reconfiguration