drivers/netdev_eth: Use NETOPT_MAX_DPU_SIZE

Updated netdev_eth to use NETOPT_MAX_DPU_SIZE instead of the deprecated
NETOPT_MAX_PACKET_SIZE

drivers/netdev_eth/netdev_eth.c

@@ -63,7 +63,7 @@ int netdev_eth_get(netdev_t *dev, netopt_t opt, void *value, size_t max_len)
                 res = sizeof(uint16_t);
                 break;
             }
-        case NETOPT_MAX_PACKET_SIZE:
+        case NETOPT_MAX_PDU_SIZE:
             {
                 assert(max_len >= 2);
                 uint16_t *val = (uint16_t*) value;