qeth: Fix scatter-gather regression

This patch fixes a scatter-gather regression introduced with

commit 5640f768 net: use a per task frag allocator

Now the qeth driver can cope with bigger framents and split a fragment in
sub framents if required.

Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>