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: I9573958fbe308c170c203a6a50a94d0540f3e7ce
Signed-off-by: Xiaoyu Ye <benyxy@codeaurora.org>