honor ifp->nerr

src/drivers/stm32h.c

@@ -213,6 +213,7 @@ static size_t mg_tcpip_driver_stm32h_tx(const void *buf, size_t len,
     MG_ERROR(("Frame too big, %ld", (long) len));
     len = 0;  // Frame is too big
   } else if ((s_txdesc[s_txno][3] & BIT(31))) {
+    ifp->nerr++;
     MG_ERROR(("No free descriptors: %u %08X %08X %08X", s_txno,
               s_txdesc[s_txno][3], ETH->DMACSR, ETH->DMACTCR));
     for (int i = 0; i < ETH_DESC_CNT; i++) MG_ERROR(("%08X", s_txdesc[i][3]));