audio: free and assign NULL to global static device pointer

From: Preetam Singh Ranawat <apranawat@codeaurora.org>
Date: Mon, 24 Jun 2019 15:11:28 +0530
Subject: audio: free and assign NULL to global static device pointer

 -free and assign NULL to global static device pointer instead of local
  pointer to avoid use after free issue.

Bug: 142267478
Test: manual
Change-Id: I6f64fe0f6034844279c9a481726426dc5b989b41
(cherry picked from commit 73d6b599)