first descriptor in TSO case. Otherwise controller can generate bad
frames during TSO. To address it, make sure to pull up ethernet +
IP + TCP header with options in first buffer. Also ensure the
buffer length of the first descriptor for TSO covers entire ethernet
+ IP + TCP with options and setup additional Tx descriptor if the
first buffer includes TCP payload.
Tested by: Amar Takhar <verm <> darkbeer dot org >
MFC after: 1 week