s390/qeth: clarify default cmd callback

Current code makes it look like qeth_send_control_data_cb() is some
sort of default callback for all cmds. But in practice, it is only used
for half of the cmd buffers we issue.
Reduce the confusion by only setting this callback for cmds that
actually want it, and while at it give the callback a name that matches
the established naming scheme.

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