qeth: Rework qeth_dbf_longtext

This patch decouples qeth_dbf_longtext from qeth's static debug array. The
function only uses one member anyway.

Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>