cxgb4: Fix error codes in c4iw_create_cq()

If one of these kmalloc() calls fails then we return ERR_PTR(0) which is
NULL.  It results in a NULL dereference in the callers.

Fixes: cfdda9d7 ("RDMA/cxgb4: Add driver for Chelsio T4 RNIC")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>