netxen: refactor tso code

o move all tso / checksum offload code into netxen_tso_check().
o optimize the tso header copy into simple loop.
o clean up unnecessary unions from cmd_desc_type0 struct.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>