[efi] Enumerate PCI BARs in same order as SnpDxe

The UEFI specification has an implicit and demonstrably incorrect
requirement (in the Mem_IO() calling convention) that any UNDI network
device has at most one memory BAR and one I/O BAR.

Some UEFI platforms have been observed to report the existence of
non-existent additional I/O BARs, causing iPXE to select the wrong
BAR.  This problem does not affect the SnpDxe driver, since that
driver will always choose the lowest numbered existent BAR of each
type.

Adjust iPXE's behaviour to match that of SnpDxe, i.e. to always select
the lowest numbered BAR(s).

Debugged-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Debugged-by: Adklei <adklei@realtek.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/efi/nii.c

 		EFI_ACPI_ADDRESS_SPACE_DESCRIPTOR *acpi;
 		void *resource;
 	} desc;
-	unsigned int bar;
+	int bar;
 	EFI_STATUS efirc;
 	int rc;
 
 	/* Identify memory and I/O BARs */
 	nii->mem_bar = PCI_MAX_BAR;
 	nii->io_bar = PCI_MAX_BAR;
-	for ( bar = 0 ; bar < PCI_MAX_BAR ; bar++ ) {
+	for ( bar = ( PCI_MAX_BAR - 1 ) ; bar >= 0 ; bar-- ) {
 		efirc = nii->pci_io->GetBarAttributes ( nii->pci_io, bar, NULL,
 							&desc.resource );
 		if ( efirc == EFI_UNSUPPORTED ) {