When STP state is set to DISABLED the port is assumed to be inactive, but
currently we forward packets ingressing through it.

Instead, set the port's STP state in hardware to DISCARDING, which means
it doesn't forward packets or perform any learning, but it does trap
control packets. However, these packets will be dropped by bridge code,
which results in the expected behavior.

Fixes: 56ade8fe ("mlxsw: spectrum: Add initial support for Spectrum ASIC")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>