mlxsw: spectrum_switchdev: Break smid write function

Break the smid write function into two, one that cleans the ports that
might be still written there and one that changes an exiting mid entry.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>