s390/cio: clear timer when terminating driver I/O

When we terminate driver I/O (because we need to stop using a certain
channel path) we also need to ensure that a timer (which may have been
set up using ccw_device_start_timeout) is cleared.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>