TSO engine of L1 requires a separate DMA descriptor for TCP

TSO engine of L1 requires a separate DMA descriptor for TCP
payload.  This means driver has to split a TX buffer into two
pieces of TX buffers when the TX buffer contains both
ethernet/IP/TCP header and partial TCP payload.  The controller
does not require all header should be in a TX buffer but driver
forced it to compute IP/TCP header size/offset which is required
parameter to configure DMA descriptor for TSO.
While here, slightly reorder DMA descriptor setup to enhance
readability and remove unnecessary code for TSO(upper stack never
requests TSO when the frame length is less than or equal to MTU).

Reported by:	Yamagi Burmeister <lists <> yamagi dot org>
Tested by:	Yamagi Burmeister <lists <> yamagi dot org>
MFC After:	1 week