mlxsw: core: Change order of operations in removal path

We call bus->init() before allocating 'lag.mapping'. Change the order of
operations in removal path to reflect that.

This makes the error path of mlxsw_core_bus_device_register() symmetric
with mlxsw_core_bus_device_unregister().

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>