IB/ipath: Fix RDMA reads of length zero and error handling

Fix RDMA read response length checking for RDMA_READ_RESPONSE_ONLY to
allow a zero length response.  RDMA read responses which don't match
the expected length or occur in response to some other operation
should generate a completion queue error (see table 56, ch. 9.9.2.3 in
the IB spec).

Signed-off-by: Bryan O'Sullivan <bryan.osullivan@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>