f_mount() API is used in both mount and unmount functions but

when mount called f_mount() is doing both register filesystem and
also initializing underlying block device, but in case of umount calls
f_mount is doing deregister filesystem and it is not deinitializing block device
so added the deint() call in umount API to deinitialize underlying block device.

Author: rajkan01

storage/filesystem/fat/source/FATFileSystem.cpp

@@ -330,8 +330,14 @@ int FATFileSystem::unmount()
     }
 
     FRESULT res = f_mount(NULL, _fsid, 0);
-    _ffs[_id] = NULL;
-    _id = -1;
+    if (!res) {
+        int err = _ffs[_id]->deinit();
+        if (err) {
+            return err;
+        }
+        _ffs[_id] = NULL;
+        _id = -1;
+    }
     unlock();
     return fat_error_remap(res);
 }