dasd: fix unresponsive device during format

If path events occur the formatting process stucks because path
events may flush format requests from the queue.

Kick the format process after path events are handled.

Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>