mlxsw: switchx2: Check if port is usable before calling port create

Do it in a same way we do it in spectrum. Check if port is usable first
and only in that case create a port instance.

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