mlxsw: spectrum_acl: Push code start/end from mlxsw_sp_acl_tcam_vregion_migrate()

Push code from the beginning and end of function
mlxsw_sp_acl_tcam_vregion_migrate() into rehash_start()/end() functions.
Then all the things needed to be done before and after the actual
migration process will be grouped together.

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>