qeth: Make s390dbf card entries persistent

As of now, s390dbf entries for the cards are discarded as soon as the
device is removed. However, this will also bar us of all chances of
getting valuable debug information after a device has been removed.
This patch will keep the s390dbf entries around until the qeth module
is removed.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Reviewed-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>