s390/qeth: unify the tracking of active cmds on ccw device

We have one field to track _whether_ a cmd is active on a ccw device
('irq_pending'), and one to track _which_ cmd it is ('active_cmd').

Get rid of the irq_pending field, by testing active_cmd for NULL.

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