net: thunderbolt: Fix TCPv6 GSO checksum calculation

Alex reported that running ssh over IPv6 does not work with
Thunderbolt/USB4 networking driver. The reason for that is that driver
should call skb_is_gso() before calling skb_is_gso_v6(), and it should
not return false after calculates the checksum successfully. This probably
was a copy paste error from the original driver where it was done properly.

Reported-by: Alex Balcanquall <alex@alexbal.com>
Fixes: e69b6c02 ("net: Add support for networking over Thunderbolt cable")
Cc: stable@vger.kernel.org
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>