mlxsw: spectrum: Disable learning according to STP state

When port is put into LISTENING state it shouldn't populate the FDB, so
set the port's STP state in hardware to DISCARDING instead of LEARNING.
It will therefore keep listening to BPDU packets, but discard other
non-control packets and won't perform any learning.

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>