s390x/css: start with cleared cstat/dstat

When executing the start function, we should start with a clear state
regarding subchannel and device status; it is easy to forget updating one
of them after the ccw has been processed.

Note that we don't need to care about resetting the various control
fields: They are cleared by tsch(), and if they were still pending,
we wouldn't be able to execute the start function in the first
place.

Also note that we don't want to clear cstat/dstat if a suspended
subchannel is resumed.

This fixes a bug where we would continue to present channel-program
check in cstat even though later ccw requests for the subchannel
finished without error (i.e. cstat should be 0).

Cc: qemu-stable@nongnu.org
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
