netxen: fix ipv6 offload and tx cleanup

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