octeontx2-pf: Fix TSOv6 offload

commit de678ca3 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>