o fix the ip/tcp hdr offset in tx descriptors for ipv6.
o cleanup xmit function, move the tso checks into separate function,
  this reduces unnecessary endian conversions back and forth.
o optimize macros to initialize tx descriptors.
Signed-off-by: NDhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>