mlxsw: acl: Fix mlxsw_afa_block_commit error path

No rollback is needed since the chain is in consistent state and
mlxsw_afa_block_destroy() will take care of putting it away. So remove
the one we have now which is wrong. Also move the set of 'finished' flag
to the beginning of the function, because the block is certainly unusable
for future action addition no matter if the function succeeds or not.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 4cda7d8d ("mlxsw: core: Introduce flexible actions support")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>