IB/mlx4: Fix incorrect cq flushing in error state

When handling a device internal error, the driver is responsible to
drain the completion queue with flush errors.

In case a completion queue was assigned to multiple send queues, the
driver iterates over the send queues and generates flush errors of
inflight wqes. The driver must correctly pass the wc array with an
offset as a result of the previous send queue iteration. Not doing so
will overwrite previously set completions and return a wrong number
of polled completions which includes ones which were not correctly set.

Fixes: 35f05dab

 (IB/mlx4: Reset flow support for IB kernel ULPs)
Signed-off-by: Ariel Nahum <arieln@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Cc: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>