cxgbe tom: Restore support for zerocopy TCP receive for aio_read()

The commit to introduce TCP_USE_DDP support had a couple of bugs that
broke support for zerocopy receive via aio_read(). First, the length
and offset arguments to mk_update_tcb_for_ddp() were reversed which
prevented DDP from working. Second, the AIO state in the toep was
initialized too late when the first aio_read() request was queued.

Reported by: Harshavardhan Tanneru @ Chelsio
Fixes: eba13bbc37ab cxgbe: Support TCP_USE_DDP on offloaded TOE connections
MFC after: 1 week
Sponsored by: Chelsio Communications

(cherry picked from commit 70693a45381b687e40ea30710aa38cb9f24b6b02)