MAINTAINERS: Update S390 PCI Maintainer

As discussed previously with Collin, I will take over maintaining
s390 pci.
Signed-off-by: NMatthew Rosato <mjrosato@linux.ibm.com>
Message-Id: <1569590461-12562-1-git-send-email-mjrosato@linux.ibm.com>
Acked-by: NCollin Walling <walling@linux.ibm.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>

MAINTAINERS

@@ -1206,7 +1206,7 @@ T: git https://github.com/borntraeger/qemu.git s390-next
 L: qemu-s390x@nongnu.org
 
 S390 PCI
-M: Collin Walling <walling@linux.ibm.com>
+M: Matthew Rosato <mjrosato@linux.ibm.com>
 S: Supported
 F: hw/s390x/s390-pci*
 L: qemu-s390x@nongnu.org