s390/qeth: keep cmd alive after IO completion

Current code releases the cmd struct after its initial IO has completed.
Any reply processing is done independently, using a separate qeth_reply
struct.
In preparation for merging the cmd and reply structs together, take an
additional reference on the cmd object so that it stays around all the
way until qeth_send_control_data() returns.

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