s390/pci: fix MSI message data

After recent changes the MSI message data needs to specify the
function-relative IRQ number.

Reported-and-tested-by: Alexander Schmidt <alexs@linux.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>