MAINTAINERS: Update s390/pci maintainer

Update maintainer entry for s390 PCI subsystem.
Sebastian has left IBM, Niklas takes over maintainership.

Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Acked-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>