s390/qeth: propagate length of processed cmd IO data to callback

When an cmd IO completes in qeth_irq(), calculate how much data was
processed by the device and pass this value to the cmd's callback.

This allows cmds that retrieve data from the device to check whether
sufficient data was received, so we do that in qeth_read_conf_data_cb().

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