cpu/stm32/periph_eth: fix typo in initialization code

A single character type resulted in way fewer TX descriptors being
available than allocated. Not only resulted this in wasting memory,
but also when more iolist chunks than descriptors are send, the

```C
    assert(iolist_count(iolist) <= ETH_TX_DESCRIPTOR_COUNT);
```

does not trigger. As a result, old TX descriptors are being overwritten
in this case.

(cherry picked from commit 82fbe08)

cpu/stm32/periph/eth.c

@@ -242,7 +242,7 @@ static void _init_dma_descriptors(void)
     for (i = 0; i < ETH_TX_DESCRIPTOR_COUNT - 1; i++) {
         tx_desc[i].desc_next = &tx_desc[i + 1];
     }
-    tx_desc[ETH_RX_DESCRIPTOR_COUNT - 1].desc_next = &tx_desc[0];
+    tx_desc[ETH_TX_DESCRIPTOR_COUNT - 1].desc_next = &tx_desc[0];
 
     rx_curr = &rx_desc[0];
     tx_curr = &tx_desc[0];