Fix comparison of integers of different signs

There is a chance of compiling error that the second argument is compiled as "int" while the first one is "unsigned int".
Modify it to "256u" to avoid the potential compiling issue.

Error log (partial):
  ./base/logging.h:592:32: error: comparison of integers of different signs: 'const unsigned int' and 'const int' [-Werror,-Wsign-compare]
  DEFINE_CHECK_OP_IMPL(Check_LT, < )
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
  ./third_party/tensorflow/core/framework/tensor_shape.h:203:5: note: in instantiation of function template specialization 'Check_LTImpl<unsigned int, int>' requested here
      DCHECK_LT(static_cast<uint32>(dt), 256);
      ^
Change: 132219253