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 <[e-mail]>
Signed-off-by: Stefan Richter <[e-mail]>

drivers/firewire/fw-transaction.c

···
 	int ext_tcode;
 
 	if (tcode > 0x10) {
-		ext_tcode = tcode - 0x10;
+		ext_tcode = tcode & ~0x10;
 		tcode = TCODE_LOCK_REQUEST;
 	} else
 		ext_tcode = 0;