lwip: Stop dropping long TCP segments

If lwIP placed more than 2 pbufs in a TCP segment, the ethernet driver
would fail to send it as it didn't have enough Tx descriptors.  The
maximum number of pbufs outstanding for transmit that lwIP keeps is
defined by the TCP_SND_QUEUELEN macro.  I modifed the value of
LPC_NUM_BUFF_TXDESCS to take advantage of this lwIP value.  The +1
takes into account that LPC_EMAC->TxProduceIndex ==
LPC->TxConsumeIndex is reserved for indicating that the queue is empty
so a full queue uses one less than the maximum count.

Author: adamgreen

libraries/net/eth/lwip-eth/arch/lpc_emac_config.h

@@ -86,7 +86,7 @@ extern "C"
 /** \brief  Defines the number of descriptors used for TX. Must
  *          be a minimum value of 2.
  */
-#define LPC_NUM_BUFF_TXDESCS 3
+#define LPC_NUM_BUFF_TXDESCS (TCP_SND_QUEUELEN + 1)
 
 /** \brief  Set this define to 1 to enable bounce buffers for transmit pbufs
  *          that cannot be sent via the zero-copy method. Some chained pbufs