s390/dasd: fix unresponsive device after all channel paths were lost

Failfast bit was set incorrectly.
Use set_bit to enable failfast.

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