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.

CRs-Fixed: 551574
Change-Id: Ib641c80ebe2acc06396c3f36eb551ed6d3271cea
Signed-off-by: Kenneth Westfield <kwestfie@codeaurora.org>