[efi] Make EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL optional

Some UEFI systems (observed with a Hyper-V virtual machine) do not
provide EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL.  Make this an optional
protocol (and fail any attempts to access PCI configuration space via
the root bridge if the protocol is missing).

Reported-by: Colin Blacker <Colin.Blacker@computerplanet.co.uk>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_pci.c

@@ -59,7 +59,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 
 /** PCI root bridge I/O protocol */
 static EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL *efipci;
-EFI_REQUIRE_PROTOCOL ( EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL, &efipci );
+EFI_REQUEST_PROTOCOL ( EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL, &efipci );
 
 static unsigned long efipci_address ( struct pci_device *pci,
 				      unsigned long location ) {
@@ -74,6 +74,9 @@ int efipci_read ( struct pci_device *pci, unsigned long location,
 	EFI_STATUS efirc;
 	int rc;
 
+	if ( ! efipci )
+		return -ENOTSUP;
+
 	if ( ( efirc = efipci->Pci.Read ( efipci, EFIPCI_WIDTH ( location ),
 					  efipci_address ( pci, location ), 1,
 					  value ) ) != 0 ) {
@@ -92,6 +95,9 @@ int efipci_write ( struct pci_device *pci, unsigned long location,
 	EFI_STATUS efirc;
 	int rc;
 
+	if ( ! efipci )
+		return -ENOTSUP;
+
 	if ( ( efirc = efipci->Pci.Write ( efipci, EFIPCI_WIDTH ( location ),
 					   efipci_address ( pci, location ), 1,
 					   &value ) ) != 0 ) {