net/mlx5e: Prevent adding the same vxlan port

Do not allow the same vxlan udp port to be added to the device more than
once.

Fixes: b3f63c3d

 ("net/mlx5e: Add netdev support for VXLAN tunneling")
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>