PCI: xgene: Do PCI error check on own line & keep return value

Instead of an "if" condition with a line split, use the usual error
handling pattern with a separate variable to improve readability.

pci_generic_config_read32() already returns either PCIBIOS_SUCCESSFUL or
PCIBIOS_DEVICE_NOT_FOUND so it is enough to simply return its return value
when ret != PCIBIOS_SUCCESSFUL.

No functional changes intended.

Link: https://lore.kernel.org/r/20230911125354.25501-6-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>