Add explicit casts to avoid compiler warnings

[hpkfft]

As seen in https://github.com/wjakob/nanobind/actions/runs/22058320272/job/63731838031
GCC complains, comparison of constant ‘1’ with boolean expression is always false [-Wbool-compare]

Also, MinGW complains:

D:/a/nanobind/nanobind/src/nb_ndarray.cpp:274:27: warning: comparison of integer expressions of different signedness: 'long int' and 'const uint32_t' {aka 'const unsigned int'} [-Wsign-compare]
  274 |     if (max_major_version >= dlpack::major_version)
      |         ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

This PR makes the compilers happy.

[wjakob]

Thanks!