uefi: Fix BopEfiGetDebugDevice handle iteration

In issue #149 on GitHub, jparris noticed that BopEfiGetDebugDevice was
trying to iterate over all the handles that responded to the
EfiSerialIoProtocolGuid, but was always calling HandleProtocol on the
first element in the handle array.

The fix is simply to call HandleProtocol using the iterator index.

boot/lib/efi/dbgser.c

@@ -244,7 +244,7 @@ Return Value:
         // Get the serial I/O protocol.
         //
 
-        EfiStatus = BopEfiHandleProtocol(Handles[0],
+        EfiStatus = BopEfiHandleProtocol(Handles[HandleIndex],
                                          &BoEfiSerialIoProtocolGuid,
                                          (VOID **)&SerialIo);