s390/stp: fix todoff size

The size of the TOD offset field in the stp info response is 64 bits.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>