s390/3270: fix crash with multiple reset device requests

If the 3270 device is detached the initial reset device request will
stays pending until the device is operational. A second reset device
call will reuse the same request structure which will cause an oops.
Add a check to see if the reset device request is already pending
and do nothing in this case.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>