mlxsw: Add enumerator for 'config_profile.flood_mode'

Currently magic constant is used for setting 'flood_mode' as part of
config profile.

As preparation for unified bridge model, which will require different
'flood_mode', add a dedicated enumerator for this field and use it as
part of 'struct mlxsw_config_profile'.

Add the relevant value for unified bridge model.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>