qdio: Keep device-specific dbf entries

Keep the per-device dbf entries until module is removed, with
proper error checking for debug feature setup.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>