net/mlx5e: Make sure the FW max encap size is enough for ipv6 tunnels

Otherwise the code that fills the ipv6 encapsulation headers could be writing
beyond the allocated headers buffer.

Fixes: ce99f6b9

 ('net/mlx5e: Support SRIOV TC encapsulation offloads for IPv6 tunnels')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>