mlxsw: spectrum_acl: Refactor vregion association code

Refactor existing _vchunk_assoc/_vchunk_deassoc() functions into
_vregion_get()/_vregion_put() to make the code simpler and prepared for
vregion locking.

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>