dsp: unlock the mutex before return to avoid potential deadlock

Function "q6asm_set_shared_circ_buff" has a potential scenario that
can cause deadlock due to a missing call to unlock mutex before return.
Avoid this potential deadlock by calling the mutex unlock just before
the return of function "q6asm_set_shared_circ_buff".

Change-Id: Ic454eb5eb57c7616d891277c09e0e3bd37e63cd2
Signed-off-by: Xiaoyu Ye <benyxy@codeaurora.org>