qeth: fill netdevice->gso_* attributes accurately

Use QETH_MAX_BUFFER_ELEMENTS(card) instead of constant 16.
Also fill gso_max_segs and gso_min_segs.

Signed-off-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>