firewire: move rcode_string() to core

There is nothing audio-specific about the rcode_string() helper, so move
it from snd-firewire-lib into firewire-core to allow other code to use it.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (fixed sound/firewire/cmp.c)

sound/firewire/cmp.c

@@ -84,7 +84,7 @@ static int pcr_modify(struct cmp_connection *c,
 	return 0;
 
 io_error:
-	cmp_error(c, "transaction failed: %s\n", rcode_string(rcode));
+	cmp_error(c, "transaction failed: %s\n", fw_rcode_string(rcode));
 	return -EIO;
 
 bus_reset: