s390/qeth: command-chain the IDX sequence

The current IDX sequence first sends one WRITE cmd to activate the
device, and then sends a second cmd that READs the response.

Using qeth_alloc_cmd(), we can combine this into a single IO with two
command-chained CCWs.

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