s390/qeth: remove qeth_wait_for_buffer()

The basic MPC initialization sequence is strictly sequential, and
waiting for an available cmd buffer should never be necessary.
So this change only affects the OSN path, where dangling waiters on an
unbounded wait_event() are not desirable. Switch to qeth_get_buffers(),
and let OSN callers deal with -ENOMEM.

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