mlxsw: spectrum_switchdev: Break mid deletion into two function

Break mid deletion into two function, so it will be possible in the future
to delete a mid entry for other reasons then switchdev command (like port
deletion).

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>