net: stmmac: enable TSO for IPv6

There is nothing in the IP that prevents us from enabling TSO for IPv6.

Before patch:
ftp fe80::2aa:bbff:fecc:1336%eth0
ftp> get /dev/zero
882512708 bytes received in 00:14 (56.11 MiB/s)

After patch:
ftp fe80::2aa:bbff:fecc:1336%eth0
ftp> get /dev/zero
1203326784 bytes received in 00:12 (94.52 MiB/s)

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>