net: chelsio: cxgb4vf: Fix an error code in cxgb4vf_pci_probe()

During the process of driver probing, probe function should return < 0
for failure, otherwise kernel will treat value == 0 as success.

Therefore, we should set err to -EINVAL when
adapter->registered_device_map is NULL. Otherwise kernel will assume
that driver has been successfully probed and will cause unexpected
errors.

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>