net/mlx5e: Protect en header file from redefinitions

add ifndef to en.h. needed for upcoming vxlan patchset.

Signed-off-by: Matthew Finlay <matt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>