rdma/cxgb4: Simplify a structure initialization

This patch avoids that sparse reports the following warning:

drivers/infiniband/hw/cxgb4/qp.c:2269:34: warning: Using plain integer as NULL pointer

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Acked-by: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>