bonding: fix destructor

It is not safe to use a network device destructor that is a function in
the module, since it can be called after module is unloaded if sysfs
handle is open.

When eventually using netlink, the device cleanup code needs to be done
via uninit function.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>