alpha: Streamline convoluted PCI error handling

miata_map_irq() handles PCI device and read config related errors in a
conditional block that is more complex than necessary.

Streamline the code flow and error handling.

No functional changes intended.

Link: https://lore.kernel.org/r/20230911125354.25501-2-ilpo.jarvinen@linux.intel.com


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