f_mount() is doing both initializing (via find_volume() -> disk_initialize() where disk_initialize api calls Mbed OS init() function)
underlying block device and register filesystem but in case of "unmount" calls this f_mount is doing deregister filesystem
and missed to a deinitializing block device. So added the Mbed OS "deint()" call in unmount API to deinitialize block device
as f_mount API doesn't have a way to call deinit().