net/mlx5: E-switch, Introduce and use eswitch support check helper

Introduce an helper routine to get esw from a devlink device and use it
at eswitch callbacks and in subsequent patch.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>