net/mlx4_en: Prepare ethtool private flags to support more flags

Currently we support only one ethtool private flag. Prepare
mlx4_en_set_priv_flags function to support more than one private flag.
Will be used in the next patch to support hardware accelerated 802.1ad
vlan.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>