mlxsw: spectrum_span: Do not dereference destination netdev

Currently, the destination netdev to which we mirror must be a valid
netdev. However, this is going to change with the introduction of
mirroring towards the CPU port, as the CPU port does not have a backing
netdev.

Avoid dereferencing the destination netdev when it is not clear if it is
valid or not.

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