usb : dwc3: Initialize kernel stack variables properly

If kernel stack variables are not initialized properly,
there  is a chance of kernel information disclosure.
So, initialize kernel stack variables with null characters.

CRs-fixed: 2042592
Change-Id: I213c0e5c7f67824c2cecace276ff2f8f81599d51
Signed-off-by: Sai Krishna Juturi <jsaikrishna@codeaurora.org>