mlxsw: spectrum_acl: Do not invalidate already invalid ACL groups

When a new ACL group is created its region (ACL) list is initially
empty. Thus, the call to mlxsw_sp_acl_tcam_group_update() would
basically invalidate an already invalid (non-existent) group.

Remove the unnecessary call and make the function symmetric to its del()
counterpart.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>