s390/pci: handle events for unused functions

Receiving error events for a pci function that's currently not in use
will crash the kernel. For example the procedure for FW upgrades might
include:
* remove the function from Linux
* apply FW upgrade
* rescan for new functions

Receiving an event during the FW upgrade will result in a use after free
when printing the functions name. Just print "n/a" in such cases.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>