IB/ipath: Fix RDMA write with immediate copy of last packet

When the last packet of a RDMA write with immediate is received, the
next receive work queue entry ID should be used to generate a completion
entry. The code was incorrectly resetting part of the state used to copy
the last packet.

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