IAR / FATFS - Pe029 error

IAR compilation fails at `_fs = { 0 };` due to

```
[Error] FATFileSystem.cpp@285,0: [Pe029]: expected an expression
```

Changing that to memset() fixes the issue, IAR is more sensitive
in C/C++ being mixed together than other compilers.

Author: Janne Kiiskila

features/storage/filesystem/fat/FATFileSystem.cpp

@@ -282,7 +282,7 @@ extern "C" DRESULT disk_ioctl(BYTE pdrv, BYTE cmd, void *buff)
 FATFileSystem::FATFileSystem(const char *name, BlockDevice *bd)
     : FileSystem(name), _id(-1)
 {
-    _fs = { 0 };
+    memset(&_fs, 0, sizeof(_fs));
     if (bd) {
         mount(bd);
     }