bluetooth: zero-pad PIN

Bluetooth PINs always consist of 6 digits, so make sure to add leading zeros as necessary. 

https://bugzilla.gnome.org/show_bug.cgi?id=651251

https://git.gnome.org/browse/gnome-shell/commit/?h=wip/wayland&id=76616dc98c3b1d7dd73f3976af2f8d01a30a78d5