s390/time: fix clocksource steering for negative clock offsets

The TOD clock offset injected by an STP sync check can be negative.
If the resulting total tod_steering_delta gets negative the kernel
will panic.

Change the type of tod_steering_delta to a signed type.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 75c7b6f3 ("s390/time: steer clocksource on STP sync events")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>