lwip: Fix invalid checksum bug

tcp_write() would incorrectly byte swap the checksum 1 too many times
when concatenating a pbuf to an existing TCP segment if the number of
bytes in the concatenated data was odd.  I hit this issue when I tried
to reproduce a lost segment issue reported by a mbed user in this forum
thread: http://mbed.org/forum/mbed/topic/4354/?page=2#comment-22657
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.
Merge pull request #101 from adamgreen/netFixLargeSegmentIssues

net: Fix a couple of large TCP segment issues