net/mlx5: Kconfig: Fix MLX5_EN/VXLAN build issue

When MLX5_EN=y MLX5_CORE=y and VXLAN=m there is a linker error for
vxlan_get_rx_port() due to the fact that VXLAN is a module. Change Kconfig
to select VXLAN when MLX5_CORE=y. When MLX5_CORE=m there is no dependency
on the value of VXLAN.

Fixes: b3f63c3d

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