msm: adsprpc: destroy mutex before file free

Destroy mutex before file free, to avoid use after free of mutex.

Change-Id: I4ff73dc17b15043eacbb299219a379bfd1a8efa6
Acked-by: Himateja Reddy <hmreddy@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>