net/mlx5e: Fix nest_level for vlan pop action

Current value of nest_level, assigned from net_device lower_level value,
does not reflect the actual number of vlan headers, needed to pop.
For ex., if we have untagged ingress traffic sended over vlan devices,
instead of one pop action, driver will perform two pop actions.
To fix that, calculate nest_level as difference between vlan device and
parent device lower_levels.

Fixes: f3b0a18b

 ("net: remove unnecessary variables and callback")
Signed-off-by: Dmytro Linkin <dmitrolin@mellanox.com>
Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>