net/mlx5e: Check netdev pointer before checking its net ns

Previously, when comparing the net namespaces, the case where the netdev
doesn't exist wasn't taken into account, and therefore can cause a crash.
In such a case, the comparing function should return false, as there is no
netdev->net to compare the devlink->net to.

Furthermore, this will result in an attempt to enter switchdev mode
without a netdev to fail, and which is the desired result as there is no
meaning in switchdev mode without a net device.

Fixes: 662404b2 ("net/mlx5e: Block entering switchdev mode with ns inconsistency")
Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Gavi Teitz <gavi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>