Several drivers already provide an empty callback so we can actually
make this optional and then remove all those empty callbacks in the
drivers.

This callback isn't needed at all by most drivers as they can remove
their allocated structures on device disconnect and not on hci
destruction.
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>