net/mlx5e: make VXLAN support conditional

VXLAN can be disabled at compile-time or it can be a loadable
module while mlx5 is built-in, which leads to a link error:

drivers/net/built-in.o: In function `mlx5e_create_netdev':
ntb_netdev.c:(.text+0x106de4): undefined reference to `vxlan_get_rx_port'

This avoids the link error and makes the vxlan code optional,
like the other ethernet drivers do as well.

Link: https://patchwork.ozlabs.org/patch/589296/


Fixes: b3f63c3d ("net/mlx5e: Add netdev support for VXLAN tunneling")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>