KVM: s390: pci: fix plain integer as NULL pointer warnings

Fix some sparse warnings that a plain integer 0 is being used instead of
NULL.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/20220915175514.167899-1-mjrosato@linux.ibm.com


Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>