s390/qeth: remove 'channel' parameter from callbacks

Each cmd buffer maintains a pointer to the IO channel that it was/will
be issued on. So when dealing with cmd buffers, we don't need to pass
around a separate channel pointer.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>