cxgb3 - use immediate data for offload Tx

Send small TX_DATA work requests as immediate data even when
there are fragments. this avoids doing multiple DMAs for
small fragmented packets.
The driver already implements this optimization for small
contiguous packets.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>