net/mlx5e: Don't attempt to dereference the ppriv struct if not being eswitch manager

The check for cpu hit statistics was not returning immediate false for
any non vport rep netdev and hence we crashed (say on mlx5 probed VFs) if
user-space tool was calling into any possible netdev in the system.

Fix that by doing a proper check before dereferencing.

Fixes: 1d447a39

 ('net/mlx5e: Extendable vport representor netdev private data')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Eli Cohen <eli@melloanox.com>
Reviewed-by: Eli Cohen <eli@melloanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>