mlxsw: spectrum_span: Support LAG under mirror-to-gretap

When resolving a path that the packet will take after being encapsulated
in mirror-to-gretap scenarios, one of the devices en route could be a
LAG. In that case, mirror to first up slave that corresponds to a front
panel port.

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>