IB/ipath: Fix race with ACK retry timeout list management

When an ACK is received, the QP is removed from the timeout list and
then if there are still pending send WQEs, the QP is put back on the
timeout list. It is possible that another post send has put the QP on
the timeout list thus, a check needs to be made before trying to do it
again or the list is corrupted.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>