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 <srinivas.kandagatla@linaro.org>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Sanyog Kale <sanyog.r.kale@intel.com>
[vkoul: edited the change log as suggested by Pierre]
Signed-off-by: Vinod Koul <vkoul@kernel.org>