mlxsw: spectrum_span: Change LAG lower selection

When offloading mirror-to-gretap, mlxsw needs to preroute the path that
the encapsulated packet will take. That path may include a LAG device
above a front panel port. So far, mlxsw resolved the path to the first
up front panel slave of the LAG interface, but that only reflects
administrative state of the port. It neglects to consider whether the
port actually has a carrier, and what the LACP state is.

So instead of checking upness of the device, check carrier state and
txability.

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