octeontx2-pf: Fix TSOv6 offload

HW adds segment size to the payload length
in the IPv6 header. Fix payload length to
just TCP header length instead of 'TCP header
size + IPv6 header size'.

Fixes: 86d74760

 ("octeontx2-pf: TCP segmentation offload support")
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: Ratheesh Kannoth <rkannoth@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>