[ACPI] fix run-time error checking in acpi_pci_irq_disable()

The 'bus' field in pci_dev structure should be checked before calling
pci_read_config_byte() because pci_bus_read_config_byte() called by
pci_read_config_byte() refers to 'bus' field.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>