s390x/pci: add length checking for pci sclp handlers

The configure/deconfigure sclp commands need a SCCB with a length of
at least 16. Indicate in the response code if this is not fulfilled.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
