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

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Ram Amrani <Ram.Amrani@cavium.com>
Cc: Michal Kalderon <Michal.Kalderon@cavium.com>
Cc: Ariel Elior <Ariel.Elior@cavium.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>