lib/cmdline_kunit: Remove a cast which are no-longer required

With some of the stricter type checking in KUnit's EXPECT macros
removed, a cast in cmdline_kunit is no longer required.

Remove the unnecessary cast, using NULL instead of (int *) to make it
clearer.

Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>