ieee1394: fix broken signed char assumption.

"ack_code" is assigned (and tested against) negative numbers, but was
declared as "char".  Which only works if "char" is signed - which it
necessarily isn't.

So make that signedness assumption specific.

drivers/ieee1394/ieee1394_core.h

@@ -38,8 +38,8 @@ struct hpsb_packet {
 
         /* These are core internal. */
         signed char tlabel;
-        char ack_code;
-        char tcode;
+	signed char ack_code;
+	unsigned char tcode;
 
         unsigned expect_response:1;
         unsigned no_waiter:1;