cxgb4: fix TSO descriptors

Commit 1704d748 ("cxgb4vf: small changes
to message processing structures/macros") was incomplete and causes cxgb4
to write bad TSO descriptors.  Fix that up by reverting the offending part
of that commit and adjusting field accesses now that they are one level
deeper.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>