ASoC: wcd934x-dsp-cntl: initialize local char array val

Due to the local char array that stores the codec dsp control command
is not initialized, an invalid command could cause the stack content
to be printed out in kernel dmesg. Initialize the array with memset.

Change-Id: I739eacb5058275c91bef67505531097cb364b93e
Signed-off-by: Xiaoyu Ye <benyxy@codeaurora.org>