msm: audio: qdsp6v2: Zero the cfg stack var

Write zeroes into the stack variable cfg to ensure
no previously written stack data is sent to
user-space.

Change-Id: Ie571949e3613c9e5a916b23e1dc19605496d9dbf
CRs-Fixed: 551574
Signed-off-by: Kenneth Westfield <kwestfie@codeaurora.org>