s390/qeth: simplify reply object handling

Current code enqueues & dequeues a reply object from the waiter list
in various places. In particular, the dequeue & enqueue in
qeth_send_control_data_cb() looks fragile - this can cause
qeth_clear_ipacmd_list() to skip the active object.
Add some helpers, and boil the logic down by giving
qeth_send_control_data() the sole responsibility to add and remove
objects.

qeth_send_control_data_cb() and qeth_clear_ipacmd_list() will now only
notify the reply object to interrupt its wait cycle. This can cause
a slight delay in the removal, but that's no concern.

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