net/mlx5: E-Switch, Enable reg c1 loopback when possible

Enable reg c1 loopback if firmware reports it's supported,
as this is needed for restoring packet metadata (e.g chain).

Also define helper to query if it is enabled.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>