asoc: msm-pcm: Fix deadlock with error case in latency_ctl_get()

Mutex lock happened  two times without unlock,which created
deadlock. Change mutex_lock() to mutex_unlock() to fix it.

Change-Id: I3731bdb202a664c3dbaea74f90dbea487350c804
Signed-off-by: Ajit Pandey <ajitp@codeaurora.org>