PCI: endpoint: Clean up error handling in vpci_scan_bus()

Smatch complains about inconsistent NULL checking in vpci_scan_bus():

    drivers/pci/endpoint/functions/pci-epf-vntb.c:1024 vpci_scan_bus() error: we previously assumed 'vpci_bus' could be null (see line 1021)

Instead of printing an error message and then crashing we should return
an error code and clean up.

Also the NULL check is reversed so it prints an error for success
instead of failure.

Fixes: e35f56bb ("PCI: endpoint: Support NTB transfer between RC and EP")
Link: https://lore.kernel.org/linux-pci/68e0f6a4-fd57-45d0-945b-0876f2c8cb86@moroto.mountain


Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>