mlxsw fails device enslavement for a number of reasons. Use the extack
facility to return an error message to the user stating why the enslave
is failing.

Messages are prefixed with "spectrum" so users know it is a constraint
imposed by the hardware driver. For example:
    $ ip li add br0.11 link br0 type vlan id 11
    $ ip li set swp11 master br0
    Error: spectrum: Enslaving a port to a device that already has an upper device is not supported.
Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>