[efi] Avoid returning uninitialised data from PCI configuration space reads

Under UEFI, reads from PCI configuration space may fail.  If this
happens, we should return all-ones (which will mimic the behaviour of
an absent PCI device).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/efi/efi_pci_api.h

 PCIAPI_INLINE ( efi, pci_read_config_byte ) ( struct pci_device *pci,
 					      unsigned int where,
 					      uint8_t *value ) {
+	*value = 0xff;
 	return efipci_read ( pci,
 			     EFIPCI_LOCATION ( where, EFIPCI_WIDTH_BYTE ),
 			     value );
 PCIAPI_INLINE ( efi, pci_read_config_word ) ( struct pci_device *pci,
 					      unsigned int where,
 					      uint16_t *value ) {
+	*value = 0xffff;
 	return efipci_read ( pci,
 			     EFIPCI_LOCATION ( where, EFIPCI_WIDTH_WORD ),
 			     value );
 PCIAPI_INLINE ( efi, pci_read_config_dword ) ( struct pci_device *pci,
 					       unsigned int where,
 					       uint32_t *value ) {
+	*value = 0xffffffffUL;
 	return efipci_read ( pci,
 			     EFIPCI_LOCATION ( where, EFIPCI_WIDTH_DWORD ),
 			     value );