mlxsw: spectrum: Bump minimum FW version to xx.2008.2018

The indicated version fixes an issue whereby the MOMTE register would by
default enable mirroring of ECN-marked traffic from all traffic classes,
once the ECN mirroring was configured. This fix is necessary for offload
of RED "ecn_mark" qevent.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>