MAINTAINERS: Update s390/cio maintainer

Update maintainer entry for s390 Common I/O Layer.
Sebastian has left IBM, Vineeth takes over maintainership.

Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Acked-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>