IB/mlx4: Fix possible missed completion event

If an erroneous CQE is polled in the first iteration (i.e. npolled ==
0), we don't update the consumer index and hence the hardware could
get a wrong notion of how many CQEs software polled.  Fix this by
unconditionally updating the doorbell record.  We could change the
check to be something like

	if (npolled || err != -EAGAIN)
		...

but it does not seem worth the effort since a posted write to memory
should not cost too much.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>