eth: mlx5: remove Kconfig co-dependency with VXLAN

eth: mlx5: remove Kconfig co-dependency with VXLAN

mlx5 has a Kconfig co-dependency on VXLAN, even tho it doesn't
call any VXLAN function (unlike mlxsw). Perhaps this dates back
to very old days when tunnel ports were fetched directly from
VXLAN.

Remove the dependency to allow MLX5=y + VXLAN=m kernel configs.
But still avoid compiling in the lib/vxlan code if VXLAN=n.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Link: https://patch.msgid.link/20250827234319.3504852-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>