IB/srpt: Preparations for adding RDMA/CM support

Introduce a union in struct srpt_rdma_ch for member variables that
depend on the type of connection manager. Avoid that error messages
report the IB/CM ID.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>