TCP header size is represented by number of 32bits words.

TCP header size is represented by number of 32bits words.
Fix the TCP header size calculation such that makes TSO engine
cache all header(ethernet/IP/TCP) bytes to its internal buffer.
While here, remove extra pull up for TCP payload.  Unlike some
em(4) controllers, fxp(4) does not require such work around for
TSO.
The two limitations are ethernet/IP/TCP header size should be less
than or equal to the size of controller's internal buffer(80 bytes)
and these header information should be found in the first fragment
of a TSO frame.