IAR / FATFS - Pe029 error

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

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

Changing that object initializer list instead, as that seems
to pass IAR compilation, too.

Changed original memset to object initialiser list based on review
feedback from Kevin Bracey.

Author: Janne Kiiskila

features/storage/filesystem/fat/FATFileSystem.cpp

@@ -280,9 +280,8 @@ extern "C" DRESULT disk_ioctl(BYTE pdrv, BYTE cmd, void *buff)
 
 // Filesystem implementation (See FATFilySystem.h)
 FATFileSystem::FATFileSystem(const char *name, BlockDevice *bd)
-    : FileSystem(name), _id(-1)
+    : FileSystem(name), _fs(), _id(-1)
 {
-    _fs = { 0 };
     if (bd) {
         mount(bd);
     }