RDMA/ocrdma: Use NULL instead of 0 to represent a pointer

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Selvin Xavier <selvin.xavier@broadcom.com>
Cc: Devesh Sharma <devesh.sharma@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>