qeth: add wake_up on write channel

To send commands on the write channel 8 buffers exist. If all
8 buffers are used, a wait is triggered on the write channel. When
such buffer are freed, a wake_up is needed. This patch adds the
missing wake_up in qeth_release_buffer().
This fix is especially important when running Communications
Controller for Linux on System z.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>