[S390] Use diagnose 308 for system reset

The diagnose 308 call is the prefered method for clearing all ongoing I/O.
Therefore if it is available we use it instead of doing a manual reset.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>