be2net: fix TSO6/GSO issue causing TX-stall on Lancer/BEx

IPv6 TSO requests with extension hdrs are a problem to the
Lancer and BEx chips. Workaround is to disable TSO6 feature
for such packets.

Also in Lancer chips, MSS less than 256 was resulting in TX stall.
Fix this by disabling GSO when MSS less than 256.

Signed-off-by: Suresh Reddy <suresh.reddy@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>