dokan: cast variable to the expected type before comparison

dokan: cast variable to the expected type before comparison

to fix the FTBFS due to following warning:

```
/home/jenkins-build/build/workspace/ceph-windows-pull-requests/ceph/build.deps/src/dokany/dokan/dokan.h:723:22: error: narrowing conversion of '-1' from 'int' to 'long unsigned int' [-Wnarrowing]
  723 | #define DOKAN_ERROR -1
      |                      ^
```

also, clean up the following warning:

```
/home/jenkins-build/build/workspace/ceph-windows-pull-requests/ceph/src/dokan/dbg.cc:142:62: warning: NULL used in arithmetic [-Wpointer-arith]
  142 |   o << "\n\tIsDirectory: " << (DokanFileInfo->IsDirectory != NULL);
      |
```

Signed-off-by: Kefu Chai <tchaikov@gmail.com>