The GPIO event descriptor was leaking kernel stack to
userspace because we don't zero the variable before
use. Ooops. Fix this.

Cc: stable@vger.kernel.org
Reported-by: NArnd Bergmann <arnd@arndb.de>
Reviewed-by: NBartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>