s390/qeth: convert RCD code to common IO infrastructure

The RCD code is the last remaining IO path that doesn't use the
qeth_send_control_data() infrastructure. Doing so allows us to remove
all sorts of custom state machinery and logic in the IRQ handler.

Instead of introducing statically allocated cmd buffers for this single
IO on the data channel, use the new qeth_alloc_cmd() helper.

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