RDMA/usnic: Fix passing zero to PTR_ERR in usnic_ib_pci_probe()

drivers/infiniband/hw/usnic/usnic_ib_main.c:590
usnic_ib_pci_probe() warn: passing zero to 'PTR_ERR'

Make usnic_ib_device_add() return NULL on fail path, also remove
useless NULL check for usnic_ib_discover_pf()

Fixes: e3cf00d0a87f ("IB/usnic: Add Cisco VIC low-level hardware driver")
Link: https://patch.msgid.link/r/[e-mail]
Signed-off-by: Yue Haibing <[e-mail]>
Reviewed-by: Zhu Yanjun <[e-mail]>
Reviewed-by: Jason Gunthorpe <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/hw/usnic/usnic_ib_main.c

···
 	if (!us_ibdev) {
 		usnic_err("Device %s context alloc failed\n",
 				netdev_name(pci_get_drvdata(dev)));
-		return ERR_PTR(-EFAULT);
+		return NULL;
 	}
 
 	us_ibdev->ufdev = usnic_fwd_dev_alloc(dev);
···
 	}
 
 	us_ibdev = usnic_ib_device_add(parent_pci);
-	if (IS_ERR_OR_NULL(us_ibdev)) {
-		us_ibdev = us_ibdev ? us_ibdev : ERR_PTR(-EFAULT);
+	if (!us_ibdev) {
+		us_ibdev = ERR_PTR(-EFAULT);
 		goto out;
 	}
 
···
 	}
 
 	pf = usnic_ib_discover_pf(vf->vnic);
-	if (IS_ERR_OR_NULL(pf)) {
-		usnic_err("Failed to discover pf of vnic %s with err%ld\n",
-				pci_name(pdev), PTR_ERR(pf));
-		err = pf ? PTR_ERR(pf) : -EFAULT;
+	if (IS_ERR(pf)) {
+		err = PTR_ERR(pf);
+		usnic_err("Failed to discover pf of vnic %s with err%d\n",
+				pci_name(pdev), err);
 		goto out_clean_vnic;
 	}