qeth: clean up condition when tso is used

Make conditions under which TSO is activated more stringent.
Make calculation of SBALEs required for the skb more accurate.

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>