soundwire: stream: fix bad unlock balance

the msg lock is taken for multi-link cases only but released
unconditionally, leading to an unlock balance warning for single-link usages
This patch fixes this.

=====================================
WARNING: bad unlock balance detected!
5.1.0-16506-gc1c383a6f0a2-dirty #1523 Tainted: G W
-------------------------------------
aplay/2954 is trying to release lock (&bus->msg_lock) at:
do_bank_switch+0x21c/0x480
but there are no more locks to release!

Signed-off-by: Srinivas Kandagatla <[e-mail]>
Acked-by: Pierre-Louis Bossart <[e-mail]>
Acked-by: Sanyog Kale <[e-mail]>
[vkoul: edited the change log as suggested by Pierre]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/soundwire/stream.c

···
 			goto error;
 		}
 
-		mutex_unlock(&bus->msg_lock);
+		if (bus->multi_link)
+			mutex_unlock(&bus->msg_lock);
 	}
 
 	return ret;