net/mlx5: E-Switch, Properly refer to the esw manager vport

In SmartNIC mode, the eswitch manager is not necessarily the PF
(vport 0). Use a helper function to get the correct eswitch manager
vport number and cache on the eswitch instance for fast reference.

Signed-off-by: Bodong Wang <bodong@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>