firewire: replace subtraction with bitwise and

Replace an unnecessary subtraction with a bitwise AND when determining the
value of ext_tcode in fw_fill_transaction() to save a cpu cycle or two in a
somewhat critical path.
Signed-off-by: Jarod Wilson <jwilson@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/fw-transaction.c

@@ -153,7 +153,7 @@ fw_fill_request(struct fw_packet *packet, int tcode, int tlabel,
 	int ext_tcode;
 
 	if (tcode > 0x10) {
-		ext_tcode = tcode - 0x10;
+		ext_tcode = tcode & ~0x10;
 		tcode = TCODE_LOCK_REQUEST;
 	} else
 		ext_tcode = 0;