iw_cxgb4: don't use WR keys/addrs for 0 byte reads

Only use the read sge lkey/addr and the remote rkey/addr if the
length of the read is not zero. Otherwise the read response might
be treated as the RTR read response and not delivered to the
application. Or worse Terminator hardware will fail a 0B read
if the STAG is 0 even if the read length is 0.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>