s390/qeth: clean up load/remove code for disciplines

We have two usage patterns:
1. get & ->setup() a new discipline, or
2. ->remove() & put the currently loaded one.

Add corresponding helpers that hide the internals & error handling.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>