bridge: implement missing ndo_uninit()

While the bridge driver implements an ndo_init(), it was missing a
symmetric ndo_uninit(), causing the different de-initialization
operations to be scattered around its dellink() and destructor().

Implement a symmetric ndo_uninit() and remove the overlapping operations
from its dellink() and destructor().

This is a prerequisite for the next patch, as it allows us to have a
proper cleanup upon changelink() failure during the bridge's newlink().

Fixes: b6677449 ("bridge: netlink: call br_changelink() during br_dev_newlink()")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>