net/mlx4_en: Advertize encapsulation offloads features only when VXLAN tunnel is set

Currenly we only support Large-Send and TX checksum offloads for
encapsulated traffic of type VXLAN. We must make sure to advertize
these offloads up to the stack only when VXLAN tunnel is set.

Failing to do so, would mislead the the networking stack to assume
that the driver can offload the internal TX checksum for GRE packets
and other buggy schemes.

Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>