net/mlx5e: Introduce net device priv flags infrastructure

Introduce an infrastructure for getting/setting private net device
flags.

Currently a 'nop' priv flag is added, following patches will override
the flag will actual feature specific flags.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>