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

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/20250324123132.2392077-1-yuehaibing@huawei.com
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>