asoc: wcd9360-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: I07211c4cc06b2c2df21b674e6812c4d5efc45dc4
Signed-off-by: Xiaoyu Ye <benyxy@codeaurora.org>